On Thu, Oct 17, 2019 at 12:11:16PM +0200, Uwe Kleine-König wrote:

On Thu, Oct 17, 2019 at 11:48:08AM +0200, Michal Vokáč wrote:

...

On 17. 10. 19 10:10, Uwe Kleine-König wrote:

...

A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state")) changed the semantic of pwm_get_state() and disclosed an (as it seems) common problem in lowlevel PWM drivers. By not relying on the period and duty cycle being retrievable from a disabled PWM this type of problem is worked around.

Apart from this issue only calling the pwm_get_state/pwm_apply_state combo once is also more effective.

Signed-off-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de

Hello,

There are now two reports about 01ccf903edd6 breaking a backlight. As far as I understand the problem this is a combination of the backend pwm driver yielding surprising results and the pwm-bl driver doing things more complicated than necessary.

So I guess this patch works around these problems. Still it would be interesting to find out the details in the imx driver that triggers the problem. So Adam, can you please instrument the pwm-imx27 driver to print *state at the beginning of pwm_imx27_apply() and the end of pwm_imx27_get_state() and provide the results?

Note I only compile tested this change.

Hi Uwe, I was just about to respond to the "pwm_bl on i.MX6Q broken on 5.4-RC1+" thread that I have a similar problem when you submitted this patch.

So here are my few cents:

My setup is as follows:

• imx6dl-yapp4-draco with i.MX6Solo
• backlight is controlled with inverted PWM signal
• max brightness level = 32, default brightness level set to 32 in DT.

1. Almost correct backlight behavior before 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state):

• System boots to userspace and backlight is enabled all the time from power up.

  $ dmesg | grep state [ 1.763381] get state end: -1811360608, enabled: 0

What is -1811360608? When I wrote "print *state" above, I thought about something like:

pr_info("%s: period: %u, duty: %u, polarity: %d, enabled: %d", __func__, state->period, state->duty_cycle, state->polarity, state->enabled);

A quick look into drivers/pwm/pwm-imx27.c shows that this is another driver that yields duty_cycle = 0 when the hardware is off.

It seems to me like the best recourse to fix this for now would be to patch up the drivers that return 0 when the hardware is off by caching the currently configured duty cycle.

How about the patch below?

Thierry

--- >8 --- From 15a52a7f1b910804fabd74a5882befd3f9d6bb37 Mon Sep 17 00:00:00 2001 From: Thierry Reding thierry.reding@gmail.com Date: Thu, 17 Oct 2019 12:56:00 +0200 Subject: [PATCH] pwm: imx27: Cache duty cycle register value

The hardware register containing the duty cycle value cannot be accessed when the PWM is disabled. This causes the ->get_state() callback to read back a duty cycle value of 0, which can confuse consumer drivers.

Signed-off-by: Thierry Reding thierry.reding@gmail.com --- drivers/pwm/pwm-imx27.c | 31 ++++++++++++++++++++++++------- 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c index ae11d8577f18..4113d5cd4c62 100644 --- a/drivers/pwm/pwm-imx27.c +++ b/drivers/pwm/pwm-imx27.c @@ -85,6 +85,13 @@ struct pwm_imx27_chip { struct clk *clk_per; void __iomem *mmio_base; struct pwm_chip chip; + + /* + * The driver cannot read the current duty cycle from the hardware if + * the hardware is disabled. Cache the last programmed duty cycle + * value to return in that case. + */ + unsigned int duty_cycle; };

#define to_pwm_imx27_chip(chip) container_of(chip, struct pwm_imx27_chip, chip) @@ -155,14 +162,17 @@ static void pwm_imx27_get_state(struct pwm_chip *chip, tmp = NSEC_PER_SEC * (u64)(period + 2); state->period = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);

- /* PWMSAR can be read only if PWM is enabled */ - if (state->enabled) { + /* + * PWMSAR can be read only if PWM is enabled. If the PWM is disabled, + * use the cached value. + */ + if (state->enabled) val = readl(imx->mmio_base + MX3_PWMSAR); - tmp = NSEC_PER_SEC * (u64)(val); - state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk); - } else { - state->duty_cycle = 0; - } + else + val = imx->duty_cycle; + + tmp = NSEC_PER_SEC * (u64)(val); + state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);

if (!state->enabled) pwm_imx27_clk_disable_unprepare(chip); @@ -261,6 +271,13 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm, writel(duty_cycles, imx->mmio_base + MX3_PWMSAR); writel(period_cycles, imx->mmio_base + MX3_PWMPR);

+ /* + * Store the duty cycle for future reference in cases where + * the MX3_PWMSAR register can't be read (i.e. when the PWM + * is disabled). + */ + imx->duty_cycle = duty_cycles; + cr = MX3_PWMCR_PRESCALER_SET(prescale) | MX3_PWMCR_STOPEN | MX3_PWMCR_DOZEN | MX3_PWMCR_WAITEN | FIELD_PREP(MX3_PWMCR_CLKSRC, MX3_PWMCR_CLKSRC_IPG_HIGH) |

On Thu, Oct 17, 2019 at 02:09:17PM +0200, Uwe Kleine-König wrote:

On Thu, Oct 17, 2019 at 01:11:31PM +0200, Thierry Reding wrote:

...

On Thu, Oct 17, 2019 at 12:11:16PM +0200, Uwe Kleine-König wrote:

...

On Thu, Oct 17, 2019 at 11:48:08AM +0200, Michal Vokáč wrote:

...

On 17. 10. 19 10:10, Uwe Kleine-König wrote:

...

A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state")) changed the semantic of pwm_get_state() and disclosed an (as it seems) common problem in lowlevel PWM drivers. By not relying on the period and duty cycle being retrievable from a disabled PWM this type of problem is worked around.

Apart from this issue only calling the pwm_get_state/pwm_apply_state combo once is also more effective.

Signed-off-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de

Hello,

There are now two reports about 01ccf903edd6 breaking a backlight. As far as I understand the problem this is a combination of the backend pwm driver yielding surprising results and the pwm-bl driver doing things more complicated than necessary.

So I guess this patch works around these problems. Still it would be interesting to find out the details in the imx driver that triggers the problem. So Adam, can you please instrument the pwm-imx27 driver to print *state at the beginning of pwm_imx27_apply() and the end of pwm_imx27_get_state() and provide the results?

Note I only compile tested this change.

Hi Uwe, I was just about to respond to the "pwm_bl on i.MX6Q broken on 5.4-RC1+" thread that I have a similar problem when you submitted this patch.

So here are my few cents:

My setup is as follows:

• imx6dl-yapp4-draco with i.MX6Solo
• backlight is controlled with inverted PWM signal
• max brightness level = 32, default brightness level set to 32 in DT.

1. Almost correct backlight behavior before 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state):

• System boots to userspace and backlight is enabled all the time from power up.

  $ dmesg | grep state [ 1.763381] get state end: -1811360608, enabled: 0

What is -1811360608? When I wrote "print *state" above, I thought about something like:

pr_info("%s: period: %u, duty: %u, polarity: %d, enabled: %d", __func__, state->period, state->duty_cycle, state->polarity, state->enabled);

A quick look into drivers/pwm/pwm-imx27.c shows that this is another driver that yields duty_cycle = 0 when the hardware is off.

It seems to me like the best recourse to fix this for now would be to patch up the drivers that return 0 when the hardware is off by caching the currently configured duty cycle.

How about the patch below?

Thierry

--- >8 --- From 15a52a7f1b910804fabd74a5882befd3f9d6bb37 Mon Sep 17 00:00:00 2001 From: Thierry Reding thierry.reding@gmail.com Date: Thu, 17 Oct 2019 12:56:00 +0200 Subject: [PATCH] pwm: imx27: Cache duty cycle register value

The hardware register containing the duty cycle value cannot be accessed when the PWM is disabled. This causes the ->get_state() callback to read back a duty cycle value of 0, which can confuse consumer drivers.

Signed-off-by: Thierry Reding thierry.reding@gmail.com

drivers/pwm/pwm-imx27.c | 31 ++++++++++++++++++++++++------- 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c index ae11d8577f18..4113d5cd4c62 100644 --- a/drivers/pwm/pwm-imx27.c +++ b/drivers/pwm/pwm-imx27.c @@ -85,6 +85,13 @@ struct pwm_imx27_chip { struct clk *clk_per; void __iomem *mmio_base; struct pwm_chip chip;

• /*

• * The driver cannot read the current duty cycle from the hardware if
  

• * the hardware is disabled. Cache the last programmed duty cycle
  

• * value to return in that case.
  

• */
  

• unsigned int duty_cycle;

};

#define to_pwm_imx27_chip(chip) container_of(chip, struct pwm_imx27_chip, chip) @@ -155,14 +162,17 @@ static void pwm_imx27_get_state(struct pwm_chip *chip, tmp = NSEC_PER_SEC * (u64)(period + 2); state->period = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);

• /* PWMSAR can be read only if PWM is enabled */
• if (state->enabled) {

• /*

• * PWMSAR can be read only if PWM is enabled. If the PWM is disabled,
  

• * use the cached value.
  

• */
  

• if (state->enabled) val = readl(imx->mmio_base + MX3_PWMSAR);

• tmp = NSEC_PER_SEC * (u64)(val);
  

• state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);
  

• } else {

• state->duty_cycle = 0;
  

• }

• else

• val = imx->duty_cycle;
  

• tmp = NSEC_PER_SEC * (u64)(val);

• state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);

  if (!state->enabled) pwm_imx27_clk_disable_unprepare(chip);

@@ -261,6 +271,13 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm, writel(duty_cycles, imx->mmio_base + MX3_PWMSAR); writel(period_cycles, imx->mmio_base + MX3_PWMPR);

• /*
  

•  * Store the duty cycle for future reference in cases where
  

•  * the MX3_PWMSAR register can't be read (i.e. when the PWM
  

•  * is disabled).
  

•  */
  

• imx->duty_cycle = duty_cycles;
  

I wonder if it would be more sensible to do this in the pwm core instead. Currently there are two drivers known with this problem. I wouldn't be surprised if there were more.

I've inspected all the drivers and didn't spot any beyond cros-ec and i.MX that have this problem. There's also no good way to do this in the core, because the core doesn't know whether or not the driver is capable of returning the correct duty cycle on hardare readout. So the core would have to rely on state->duty_cycle that is passed in, but then the offending commit becomes useless because the whole point was to return the state as written to hardware (rather than the software state which was being returned before that patch).

If we want to move clients to not rely on .period and .duty_cycle for a disabled PWM (do we?) a single change in the core is also beneficial compared to fixing several lowlevel drivers.

These are really two orthogonal problems. We don't currently consider enabled = 0 to be equivalent to duty_cycle = 0 at an API level. I'm not prepared to do that at this point in the release cycle either.

What this here has shown is that we have at least two drivers that don't behave the way they are supposed to according to the API and they break consumers. If they break for pwm-backlight, it's possible that they will break for other consumers as well. So the right thing to do is fix the two drivers that are broken.

After -rc1 we no longer experiment. Instead we clean up the messes we've made. We can revisit the other points once mainline is fixed.

Thierry

On Thu, Oct 17, 2019 at 03:58:25PM +0200, Michal Vokáč wrote:

On 17. 10. 19 14:59, Thierry Reding wrote:

...

On Thu, Oct 17, 2019 at 02:09:17PM +0200, Uwe Kleine-König wrote:

...

On Thu, Oct 17, 2019 at 01:11:31PM +0200, Thierry Reding wrote:

...

On Thu, Oct 17, 2019 at 12:11:16PM +0200, Uwe Kleine-König wrote:

...

On Thu, Oct 17, 2019 at 11:48:08AM +0200, Michal Vokáč wrote:

...

On 17. 10. 19 10:10, Uwe Kleine-König wrote: > A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let > pwm_get_state() return the last implemented state")) changed the > semantic of pwm_get_state() and disclosed an (as it seems) common > problem in lowlevel PWM drivers. By not relying on the period and duty > cycle being retrievable from a disabled PWM this type of problem is > worked around. > > Apart from this issue only calling the pwm_get_state/pwm_apply_state > combo once is also more effective. > > Signed-off-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de > --- > Hello, > > There are now two reports about 01ccf903edd6 breaking a backlight. As > far as I understand the problem this is a combination of the backend pwm > driver yielding surprising results and the pwm-bl driver doing things > more complicated than necessary. > > So I guess this patch works around these problems. Still it would be > interesting to find out the details in the imx driver that triggers the > problem. So Adam, can you please instrument the pwm-imx27 driver to > print *state at the beginning of pwm_imx27_apply() and the end of > pwm_imx27_get_state() and provide the results? > > Note I only compile tested this change.

Hi Uwe, I was just about to respond to the "pwm_bl on i.MX6Q broken on 5.4-RC1+" thread that I have a similar problem when you submitted this patch.

So here are my few cents:

My setup is as follows:

• imx6dl-yapp4-draco with i.MX6Solo
• backlight is controlled with inverted PWM signal
• max brightness level = 32, default brightness level set to 32 in DT.

1. Almost correct backlight behavior before 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state):

• System boots to userspace and backlight is enabled all the time from power up.

  $ dmesg | grep state [ 1.763381] get state end: -1811360608, enabled: 0

What is -1811360608? When I wrote "print *state" above, I thought about something like:

pr_info("%s: period: %u, duty: %u, polarity: %d, enabled: %d", __func__, state->period, state->duty_cycle, state->polarity, state->enabled);

A quick look into drivers/pwm/pwm-imx27.c shows that this is another driver that yields duty_cycle = 0 when the hardware is off.

It seems to me like the best recourse to fix this for now would be to patch up the drivers that return 0 when the hardware is off by caching the currently configured duty cycle.

How about the patch below?

Thierry

--- >8 --- From 15a52a7f1b910804fabd74a5882befd3f9d6bb37 Mon Sep 17 00:00:00 2001 From: Thierry Reding thierry.reding@gmail.com Date: Thu, 17 Oct 2019 12:56:00 +0200 Subject: [PATCH] pwm: imx27: Cache duty cycle register value

The hardware register containing the duty cycle value cannot be accessed when the PWM is disabled. This causes the ->get_state() callback to read back a duty cycle value of 0, which can confuse consumer drivers.

Signed-off-by: Thierry Reding thierry.reding@gmail.com

drivers/pwm/pwm-imx27.c | 31 ++++++++++++++++++++++++------- 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c index ae11d8577f18..4113d5cd4c62 100644 --- a/drivers/pwm/pwm-imx27.c +++ b/drivers/pwm/pwm-imx27.c @@ -85,6 +85,13 @@ struct pwm_imx27_chip { struct clk *clk_per; void __iomem *mmio_base; struct pwm_chip chip;

• /*

• * The driver cannot read the current duty cycle from the hardware if
  

• * the hardware is disabled. Cache the last programmed duty cycle
  

• * value to return in that case.
  

• */
  

• unsigned int duty_cycle; }; #define to_pwm_imx27_chip(chip) container_of(chip, struct pwm_imx27_chip, chip)

@@ -155,14 +162,17 @@ static void pwm_imx27_get_state(struct pwm_chip *chip, tmp = NSEC_PER_SEC * (u64)(period + 2); state->period = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);

• /* PWMSAR can be read only if PWM is enabled */
• if (state->enabled) {

• /*

• * PWMSAR can be read only if PWM is enabled. If the PWM is disabled,
  

• * use the cached value.
  

• */
  

• if (state->enabled) val = readl(imx->mmio_base + MX3_PWMSAR);

• tmp = NSEC_PER_SEC * (u64)(val);
  

• state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);
  

• } else {

• state->duty_cycle = 0;
  

• }

• else

• val = imx->duty_cycle;
  

• tmp = NSEC_PER_SEC * (u64)(val);
• state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk); if (!state->enabled) pwm_imx27_clk_disable_unprepare(chip);

@@ -261,6 +271,13 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm, writel(duty_cycles, imx->mmio_base + MX3_PWMSAR); writel(period_cycles, imx->mmio_base + MX3_PWMPR);

• /*
  

•  * Store the duty cycle for future reference in cases where
  

•  * the MX3_PWMSAR register can't be read (i.e. when the PWM
  

•  * is disabled).
  

•  */
  

• imx->duty_cycle = duty_cycles;
  

I wonder if it would be more sensible to do this in the pwm core instead. Currently there are two drivers known with this problem. I wouldn't be surprised if there were more.

I've inspected all the drivers and didn't spot any beyond cros-ec and i.MX that have this problem. There's also no good way to do this in the core, because the core doesn't know whether or not the driver is capable of returning the correct duty cycle on hardare readout. So the core would have to rely on state->duty_cycle that is passed in, but then the offending commit becomes useless because the whole point was to return the state as written to hardware (rather than the software state which was being returned before that patch).

...

If we want to move clients to not rely on .period and .duty_cycle for a disabled PWM (do we?) a single change in the core is also beneficial compared to fixing several lowlevel drivers.

These are really two orthogonal problems. We don't currently consider enabled = 0 to be equivalent to duty_cycle = 0 at an API level. I'm not prepared to do that at this point in the release cycle either.

What this here has shown is that we have at least two drivers that don't behave the way they are supposed to according to the API and they break consumers. If they break for pwm-backlight, it's possible that they will break for other consumers as well. So the right thing to do is fix the two drivers that are broken.

After -rc1 we no longer experiment. Instead we clean up the messes we've made. We can revisit the other points once mainline is fixed.

Hi Thierry, I just tried your patch with v5.4-rc3 with this result:

root@hydraco:~# dmesg | grep pwm_ [ 1.772089] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 4.938759] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 4.947431] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0

Okay... this is interesting. If I understand correctly, that first line here is where the initial hardware readout happens. The second one is the first time when the backlight is configured, so it sets period and polarity. But then for some reason when we read out after that to read what state was written... we see that actually nothing was written at all.

And we can see why in pwm_imx27_apply(): If the PWM is not enabled, we don't actually program any of the registers, so it's not a surprise that things fall apart.

[ 4.956484] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 0 [ 4.965473] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 4.974410] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 1 [ 4.988617] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 1

Backlight is on with full brightness at this stage.

root@hydraco:/sys/class/backlight/backlight# cat brightness 32

root@hydraco:/sys/class/backlight/backlight# echo 32 > brightness [ 153.386391] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 1 [ 153.398311] pwm_imx27_get_state: period: 992970, duty: 992970, polarity: 0, enabled: 1

Backlight goes down.

root@hydraco:/sys/class/backlight/backlight# echo 1 > brightness [ 168.506261] pwm_imx27_apply: period: 992970, duty: 15576, polarity: 0, enabled: 1 [ 168.518064] pwm_imx27_get_state: period: 992970, duty: 15576, polarity: 0, enabled: 1

Backlight goes up to almost full brightness.

root@hydraco:/sys/class/backlight/backlight# echo 0 > brightness [ 177.496265] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 177.507602] pwm_imx27_get_state: period: 496485, duty: 7788, polarity: 0, enabled: 0

Backlight goes up to full brightness.

So your patch does not solve my issue.

The main problem I see is incorrect polarity setting. In my DT the pwm-backlight consumer requests PWM_POLARITY_INVERTED and period 500000ns. Though after reset the PWM HW registers are configured to normal polarity. This initial setting is read out and used by the consumer instead of the DT configuration.

So the problem with the i.MX driver is that it doesn't actually write the full state to the hardware and therefore the patch that caused these things to break reads back an incomplete state. So we've basically got two options: 1) make sure the hardware state is fully written or 2) make sure that we return the cached state.

I think 2) doesn't really make sense because it is conflicts with the purpose of the ->get_state() callback. The only time where we should be returning cached data is if the hardware registers don't contain the information (as in the case of the cros-ec driver) or if we can't access it for other reasons (such as in the case of i.MX's duty cycle).

Does the attached patch help with your issue? The idea is to always write the full state to the hardware, even if period and duty cycle are unused when the PWM is disabled. That's really the kind of contract that we have added with the offending patch in the core.

It looks like all other drivers handle this more or less correctly, so if we only need to fix up cros-ec and i.MX this seems like a realistic way to fix things up. If other drivers are problematic in this regard, we should probably revert and then fix the drivers before we can apply that patch again.

Thierry

--- >8 --- From 7040f0038e04a1caa6dda5b6f675a9fdee0271f4 Mon Sep 17 00:00:00 2001 From: Thierry Reding thierry.reding@gmail.com Date: Thu, 17 Oct 2019 17:11:41 +0200 Subject: [PATCH] pwm: imx27: Unconditionally write state to hardware

The i.MX driver currently uses a shortcut and doesn't write all of the state through to the hardware when the PWM is disabled. This causes an inconsistent state to be read back by consumers with the result of them malfunctioning.

Fix this by always writing the full state through to the hardware registers so that the correct state can always be read back.

Signed-off-by: Thierry Reding thierry.reding@gmail.com --- drivers/pwm/pwm-imx27.c | 120 ++++++++++++++++++++-------------------- 1 file changed, 59 insertions(+), 61 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c index 4113d5cd4c62..59d8b1289808 100644 --- a/drivers/pwm/pwm-imx27.c +++ b/drivers/pwm/pwm-imx27.c @@ -230,70 +230,68 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,

pwm_get_state(pwm, &cstate);

- if (state->enabled) { - c = clk_get_rate(imx->clk_per); - c *= state->period; - - do_div(c, 1000000000); - period_cycles = c; - - prescale = period_cycles / 0x10000 + 1; - - period_cycles /= prescale; - c = (unsigned long long)period_cycles * state->duty_cycle; - do_div(c, state->period); - duty_cycles = c; - - /* - * according to imx pwm RM, the real period value should be - * PERIOD value in PWMPR plus 2. - */ - if (period_cycles > 2) - period_cycles -= 2; - else - period_cycles = 0; - - /* - * Wait for a free FIFO slot if the PWM is already enabled, and - * flush the FIFO if the PWM was disabled and is about to be - * enabled. - */ - if (cstate.enabled) { - pwm_imx27_wait_fifo_slot(chip, pwm); - } else { - ret = pwm_imx27_clk_prepare_enable(chip); - if (ret) - return ret; - - pwm_imx27_sw_reset(chip); - } - - writel(duty_cycles, imx->mmio_base + MX3_PWMSAR); - writel(period_cycles, imx->mmio_base + MX3_PWMPR); - - /* - * Store the duty cycle for future reference in cases where - * the MX3_PWMSAR register can't be read (i.e. when the PWM - * is disabled). - */ - imx->duty_cycle = duty_cycles; - - cr = MX3_PWMCR_PRESCALER_SET(prescale) | - MX3_PWMCR_STOPEN | MX3_PWMCR_DOZEN | MX3_PWMCR_WAITEN | - FIELD_PREP(MX3_PWMCR_CLKSRC, MX3_PWMCR_CLKSRC_IPG_HIGH) | - MX3_PWMCR_DBGEN | MX3_PWMCR_EN; - - if (state->polarity == PWM_POLARITY_INVERSED) - cr |= FIELD_PREP(MX3_PWMCR_POUTC, - MX3_PWMCR_POUTC_INVERTED); - - writel(cr, imx->mmio_base + MX3_PWMCR); - } else if (cstate.enabled) { - writel(0, imx->mmio_base + MX3_PWMCR); + c = clk_get_rate(imx->clk_per); + c *= state->period;

- pwm_imx27_clk_disable_unprepare(chip); + do_div(c, 1000000000); + period_cycles = c; + + prescale = period_cycles / 0x10000 + 1; + + period_cycles /= prescale; + c = (unsigned long long)period_cycles * state->duty_cycle; + do_div(c, state->period); + duty_cycles = c; + + /* + * according to imx pwm RM, the real period value should be PERIOD + * value in PWMPR plus 2. + */ + if (period_cycles > 2) + period_cycles -= 2; + else + period_cycles = 0; + + /* + * Wait for a free FIFO slot if the PWM is already enabled, and flush + * the FIFO if the PWM was disabled and is about to be enabled. + */ + if (cstate.enabled) { + pwm_imx27_wait_fifo_slot(chip, pwm); + } else { + ret = pwm_imx27_clk_prepare_enable(chip); + if (ret) + return ret; + + pwm_imx27_sw_reset(chip); }

+ writel(duty_cycles, imx->mmio_base + MX3_PWMSAR); + writel(period_cycles, imx->mmio_base + MX3_PWMPR); + + /* + * Store the duty cycle for future reference in cases where the + * MX3_PWMSAR register can't be read (i.e. when the PWM is disabled). + */ + imx->duty_cycle = duty_cycles; + + cr = MX3_PWMCR_PRESCALER_SET(prescale) | + MX3_PWMCR_STOPEN | MX3_PWMCR_DOZEN | MX3_PWMCR_WAITEN | + FIELD_PREP(MX3_PWMCR_CLKSRC, MX3_PWMCR_CLKSRC_IPG_HIGH) | + MX3_PWMCR_DBGEN; + + if (state->polarity == PWM_POLARITY_INVERSED) + cr |= FIELD_PREP(MX3_PWMCR_POUTC, + MX3_PWMCR_POUTC_INVERTED); + + if (state->enabled) + cr |= MX3_PWMCR_EN; + + writel(cr, imx->mmio_base + MX3_PWMCR); + + if (!state->enabled && cstate.enabled) + pwm_imx27_clk_disable_unprepare(chip); + return 0; }

On Thu, Oct 17, 2019 at 12:07:21PM -0500, Adam Ford wrote:

On Thu, Oct 17, 2019 at 10:14 AM Thierry Reding thierry.reding@gmail.com wrote:

...

On Thu, Oct 17, 2019 at 03:58:25PM +0200, Michal Vokáč wrote:

...

On 17. 10. 19 14:59, Thierry Reding wrote:

...

On Thu, Oct 17, 2019 at 02:09:17PM +0200, Uwe Kleine-König wrote:

...

On Thu, Oct 17, 2019 at 01:11:31PM +0200, Thierry Reding wrote:

...

On Thu, Oct 17, 2019 at 12:11:16PM +0200, Uwe Kleine-König wrote: > On Thu, Oct 17, 2019 at 11:48:08AM +0200, Michal Vokáč wrote: > > On 17. 10. 19 10:10, Uwe Kleine-König wrote: > > > A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let > > > pwm_get_state() return the last implemented state")) changed the > > > semantic of pwm_get_state() and disclosed an (as it seems) common > > > problem in lowlevel PWM drivers. By not relying on the period and duty > > > cycle being retrievable from a disabled PWM this type of problem is > > > worked around. > > > > > > Apart from this issue only calling the pwm_get_state/pwm_apply_state > > > combo once is also more effective. > > > > > > Signed-off-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de > > > --- > > > Hello, > > > > > > There are now two reports about 01ccf903edd6 breaking a backlight. As > > > far as I understand the problem this is a combination of the backend pwm > > > driver yielding surprising results and the pwm-bl driver doing things > > > more complicated than necessary. > > > > > > So I guess this patch works around these problems. Still it would be > > > interesting to find out the details in the imx driver that triggers the > > > problem. So Adam, can you please instrument the pwm-imx27 driver to > > > print *state at the beginning of pwm_imx27_apply() and the end of > > > pwm_imx27_get_state() and provide the results? > > > > > > Note I only compile tested this change. > > > > Hi Uwe, > > I was just about to respond to the "pwm_bl on i.MX6Q broken on 5.4-RC1+" > > thread that I have a similar problem when you submitted this patch. > > > > So here are my few cents: > > > > My setup is as follows: > > - imx6dl-yapp4-draco with i.MX6Solo > > - backlight is controlled with inverted PWM signal > > - max brightness level = 32, default brightness level set to 32 in DT. > > > > 1. Almost correct backlight behavior before 01ccf903edd6 ("pwm: Let > > pwm_get_state() return the last implemented state): > > > > - System boots to userspace and backlight is enabled all the time from > > power up. > > > > $ dmesg | grep state > > [ 1.763381] get state end: -1811360608, enabled: 0 > > What is -1811360608? When I wrote "print *state" above, I thought about > something like: > > pr_info("%s: period: %u, duty: %u, polarity: %d, enabled: %d", > __func__, state->period, state->duty_cycle, state->polarity, state->enabled); > > A quick look into drivers/pwm/pwm-imx27.c shows that this is another > driver that yields duty_cycle = 0 when the hardware is off.

It seems to me like the best recourse to fix this for now would be to patch up the drivers that return 0 when the hardware is off by caching the currently configured duty cycle.

How about the patch below?

Thierry

--- >8 --- From 15a52a7f1b910804fabd74a5882befd3f9d6bb37 Mon Sep 17 00:00:00 2001 From: Thierry Reding thierry.reding@gmail.com Date: Thu, 17 Oct 2019 12:56:00 +0200 Subject: [PATCH] pwm: imx27: Cache duty cycle register value

The hardware register containing the duty cycle value cannot be accessed when the PWM is disabled. This causes the ->get_state() callback to read back a duty cycle value of 0, which can confuse consumer drivers.

Signed-off-by: Thierry Reding thierry.reding@gmail.com

drivers/pwm/pwm-imx27.c | 31 ++++++++++++++++++++++++------- 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c index ae11d8577f18..4113d5cd4c62 100644 --- a/drivers/pwm/pwm-imx27.c +++ b/drivers/pwm/pwm-imx27.c @@ -85,6 +85,13 @@ struct pwm_imx27_chip { struct clk *clk_per; void __iomem *mmio_base; struct pwm_chip chip;

•   /*
  

•    * The driver cannot read the current duty cycle from the hardware if
  

•    * the hardware is disabled. Cache the last programmed duty cycle
  

•    * value to return in that case.
  

•    */
  

•   unsigned int duty_cycle;
  

  }; #define to_pwm_imx27_chip(chip) container_of(chip, struct pwm_imx27_chip, chip)

@@ -155,14 +162,17 @@ static void pwm_imx27_get_state(struct pwm_chip *chip, tmp = NSEC_PER_SEC * (u64)(period + 2); state->period = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);

•   /* PWMSAR can be read only if PWM is enabled */
  

•   if (state->enabled) {
  

•   /*
  

•    * PWMSAR can be read only if PWM is enabled. If the PWM is disabled,
  

•    * use the cached value.
  

•    */
  

•   if (state->enabled)
            val = readl(imx->mmio_base + MX3_PWMSAR);
  

•           tmp = NSEC_PER_SEC * (u64)(val);
  

•           state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);
  

•   } else {
  

•           state->duty_cycle = 0;
  

•   }
  

•   else
  

•           val = imx->duty_cycle;
  

•   tmp = NSEC_PER_SEC * (u64)(val);
  

•   state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);
    if (!state->enabled)
            pwm_imx27_clk_disable_unprepare(chip);
  

@@ -261,6 +271,13 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm, writel(duty_cycles, imx->mmio_base + MX3_PWMSAR); writel(period_cycles, imx->mmio_base + MX3_PWMPR);

•           /*
  

•            * Store the duty cycle for future reference in cases where
  

•            * the MX3_PWMSAR register can't be read (i.e. when the PWM
  

•            * is disabled).
  

•            */
  

•           imx->duty_cycle = duty_cycles;
  

I wonder if it would be more sensible to do this in the pwm core instead. Currently there are two drivers known with this problem. I wouldn't be surprised if there were more.

I've inspected all the drivers and didn't spot any beyond cros-ec and i.MX that have this problem. There's also no good way to do this in the core, because the core doesn't know whether or not the driver is capable of returning the correct duty cycle on hardare readout. So the core would have to rely on state->duty_cycle that is passed in, but then the offending commit becomes useless because the whole point was to return the state as written to hardware (rather than the software state which was being returned before that patch).

...

If we want to move clients to not rely on .period and .duty_cycle for a disabled PWM (do we?) a single change in the core is also beneficial compared to fixing several lowlevel drivers.

These are really two orthogonal problems. We don't currently consider enabled = 0 to be equivalent to duty_cycle = 0 at an API level. I'm not prepared to do that at this point in the release cycle either.

What this here has shown is that we have at least two drivers that don't behave the way they are supposed to according to the API and they break consumers. If they break for pwm-backlight, it's possible that they will break for other consumers as well. So the right thing to do is fix the two drivers that are broken.

After -rc1 we no longer experiment. Instead we clean up the messes we've made. We can revisit the other points once mainline is fixed.

Hi Thierry, I just tried your patch with v5.4-rc3 with this result:

root@hydraco:~# dmesg | grep pwm_ [ 1.772089] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 4.938759] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 4.947431] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0

Okay... this is interesting. If I understand correctly, that first line here is where the initial hardware readout happens. The second one is the first time when the backlight is configured, so it sets period and polarity. But then for some reason when we read out after that to read what state was written... we see that actually nothing was written at all.

And we can see why in pwm_imx27_apply(): If the PWM is not enabled, we don't actually program any of the registers, so it's not a surprise that things fall apart.

...

[ 4.956484] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 0 [ 4.965473] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 4.974410] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 1 [ 4.988617] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 1

Backlight is on with full brightness at this stage.

root@hydraco:/sys/class/backlight/backlight# cat brightness 32

root@hydraco:/sys/class/backlight/backlight# echo 32 > brightness [ 153.386391] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 1 [ 153.398311] pwm_imx27_get_state: period: 992970, duty: 992970, polarity: 0, enabled: 1

Backlight goes down.

root@hydraco:/sys/class/backlight/backlight# echo 1 > brightness [ 168.506261] pwm_imx27_apply: period: 992970, duty: 15576, polarity: 0, enabled: 1 [ 168.518064] pwm_imx27_get_state: period: 992970, duty: 15576, polarity: 0, enabled: 1

Backlight goes up to almost full brightness.

root@hydraco:/sys/class/backlight/backlight# echo 0 > brightness [ 177.496265] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 177.507602] pwm_imx27_get_state: period: 496485, duty: 7788, polarity: 0, enabled: 0

Backlight goes up to full brightness.

So your patch does not solve my issue.

The main problem I see is incorrect polarity setting. In my DT the pwm-backlight consumer requests PWM_POLARITY_INVERTED and period 500000ns. Though after reset the PWM HW registers are configured to normal polarity. This initial setting is read out and used by the consumer instead of the DT configuration.

So the problem with the i.MX driver is that it doesn't actually write the full state to the hardware and therefore the patch that caused these things to break reads back an incomplete state. So we've basically got two options: 1) make sure the hardware state is fully written or 2) make sure that we return the cached state.

I think 2) doesn't really make sense because it is conflicts with the purpose of the ->get_state() callback. The only time where we should be returning cached data is if the hardware registers don't contain the information (as in the case of the cros-ec driver) or if we can't access it for other reasons (such as in the case of i.MX's duty cycle).

Does the attached patch help with your issue? The idea is to always write the full state to the hardware, even if period and duty cycle are unused when the PWM is disabled. That's really the kind of contract that we have added with the offending patch in the core.

It looks like all other drivers handle this more or less correctly, so if we only need to fix up cros-ec and i.MX this seems like a realistic way to fix things up. If other drivers are problematic in this regard, we should probably revert and then fix the drivers before we can apply that patch again.

This patch combined with your previous patch appears to have worked. If you end up sending a patch series to fix this, go ahead and add

Tested-by: Adam Ford aford173@gmail.com #imx6q-logicpd

Excellent! Thanks for testing this. I'll wait until tomorrow to see if there's some feedback from Enric for the cros-ec change. I'll send out the total of three patches again in the hopes that those are really the only two cases that are broken.

Thierry

...

--- >8 --- From 7040f0038e04a1caa6dda5b6f675a9fdee0271f4 Mon Sep 17 00:00:00 2001 From: Thierry Reding thierry.reding@gmail.com Date: Thu, 17 Oct 2019 17:11:41 +0200 Subject: [PATCH] pwm: imx27: Unconditionally write state to hardware

The i.MX driver currently uses a shortcut and doesn't write all of the state through to the hardware when the PWM is disabled. This causes an inconsistent state to be read back by consumers with the result of them malfunctioning.

Fix this by always writing the full state through to the hardware registers so that the correct state can always be read back.

Signed-off-by: Thierry Reding thierry.reding@gmail.com

drivers/pwm/pwm-imx27.c | 120 ++++++++++++++++++++-------------------- 1 file changed, 59 insertions(+), 61 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c index 4113d5cd4c62..59d8b1289808 100644 --- a/drivers/pwm/pwm-imx27.c +++ b/drivers/pwm/pwm-imx27.c @@ -230,70 +230,68 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,

    pwm_get_state(pwm, &cstate);

•   if (state->enabled) {
  

•           c = clk_get_rate(imx->clk_per);
  

•           c *= state->period;
  

•           do_div(c, 1000000000);
  

•           period_cycles = c;
  

•           prescale = period_cycles / 0x10000 + 1;
  

•           period_cycles /= prescale;
  

•           c = (unsigned long long)period_cycles * state->duty_cycle;
  

•           do_div(c, state->period);
  

•           duty_cycles = c;
  

•           /*
  

•            * according to imx pwm RM, the real period value should be
  

•            * PERIOD value in PWMPR plus 2.
  

•            */
  

•           if (period_cycles > 2)
  

•                   period_cycles -= 2;
  

•           else
  

•                   period_cycles = 0;
  

•           /*
  

•            * Wait for a free FIFO slot if the PWM is already enabled, and
  

•            * flush the FIFO if the PWM was disabled and is about to be
  

•            * enabled.
  

•            */
  

•           if (cstate.enabled) {
  

•                   pwm_imx27_wait_fifo_slot(chip, pwm);
  

•           } else {
  

•                   ret = pwm_imx27_clk_prepare_enable(chip);
  

•                   if (ret)
  

•                           return ret;
  

•                   pwm_imx27_sw_reset(chip);
  

•           }
  

•           writel(duty_cycles, imx->mmio_base + MX3_PWMSAR);
  

•           writel(period_cycles, imx->mmio_base + MX3_PWMPR);
  

•           /*
  

•            * Store the duty cycle for future reference in cases where
  

•            * the MX3_PWMSAR register can't be read (i.e. when the PWM
  

•            * is disabled).
  

•            */
  

•           imx->duty_cycle = duty_cycles;
  

•           cr = MX3_PWMCR_PRESCALER_SET(prescale) |
  

•                MX3_PWMCR_STOPEN | MX3_PWMCR_DOZEN | MX3_PWMCR_WAITEN |
  

•                FIELD_PREP(MX3_PWMCR_CLKSRC, MX3_PWMCR_CLKSRC_IPG_HIGH) |
  

•                MX3_PWMCR_DBGEN | MX3_PWMCR_EN;
  

•           if (state->polarity == PWM_POLARITY_INVERSED)
  

•                   cr |= FIELD_PREP(MX3_PWMCR_POUTC,
  

•                                   MX3_PWMCR_POUTC_INVERTED);
  

•           writel(cr, imx->mmio_base + MX3_PWMCR);
  

•   } else if (cstate.enabled) {
  

•           writel(0, imx->mmio_base + MX3_PWMCR);
  

•   c = clk_get_rate(imx->clk_per);
  

•   c *= state->period;
  
  

•           pwm_imx27_clk_disable_unprepare(chip);
  

•   do_div(c, 1000000000);
  

•   period_cycles = c;
  

•   prescale = period_cycles / 0x10000 + 1;
  

•   period_cycles /= prescale;
  

•   c = (unsigned long long)period_cycles * state->duty_cycle;
  

•   do_div(c, state->period);
  

•   duty_cycles = c;
  

•   /*
  

•    * according to imx pwm RM, the real period value should be PERIOD
  

•    * value in PWMPR plus 2.
  

•    */
  

•   if (period_cycles > 2)
  

•           period_cycles -= 2;
  

•   else
  

•           period_cycles = 0;
  

•   /*
  

•    * Wait for a free FIFO slot if the PWM is already enabled, and flush
  

•    * the FIFO if the PWM was disabled and is about to be enabled.
  

•    */
  

•   if (cstate.enabled) {
  

•           pwm_imx27_wait_fifo_slot(chip, pwm);
  

•   } else {
  

•           ret = pwm_imx27_clk_prepare_enable(chip);
  

•           if (ret)
  

•                   return ret;
  

•           pwm_imx27_sw_reset(chip);
    }
  
  

•   writel(duty_cycles, imx->mmio_base + MX3_PWMSAR);
  

•   writel(period_cycles, imx->mmio_base + MX3_PWMPR);
  

•   /*
  

•    * Store the duty cycle for future reference in cases where the
  

•    * MX3_PWMSAR register can't be read (i.e. when the PWM is disabled).
  

•    */
  

•   imx->duty_cycle = duty_cycles;
  

•   cr = MX3_PWMCR_PRESCALER_SET(prescale) |
  

•        MX3_PWMCR_STOPEN | MX3_PWMCR_DOZEN | MX3_PWMCR_WAITEN |
  

•        FIELD_PREP(MX3_PWMCR_CLKSRC, MX3_PWMCR_CLKSRC_IPG_HIGH) |
  

•        MX3_PWMCR_DBGEN;
  

•   if (state->polarity == PWM_POLARITY_INVERSED)
  

•           cr |= FIELD_PREP(MX3_PWMCR_POUTC,
  

•                           MX3_PWMCR_POUTC_INVERTED);
  

•   if (state->enabled)
  

•           cr |= MX3_PWMCR_EN;
  

•   writel(cr, imx->mmio_base + MX3_PWMCR);
  

•   if (!state->enabled && cstate.enabled)
  

•           pwm_imx27_clk_disable_unprepare(chip);
  

•   return 0;
  

}

-- 2.23.0

On 17. 10. 19 19:44, Adam Ford wrote:

On Thu, Oct 17, 2019 at 12:13 PM Thierry Reding thierry.reding@gmail.com wrote:

...

On Thu, Oct 17, 2019 at 12:07:21PM -0500, Adam Ford wrote:

...

On Thu, Oct 17, 2019 at 10:14 AM Thierry Reding thierry.reding@gmail.com wrote:

...

On Thu, Oct 17, 2019 at 03:58:25PM +0200, Michal Vokáč wrote:

...

On 17. 10. 19 14:59, Thierry Reding wrote:

...

On Thu, Oct 17, 2019 at 02:09:17PM +0200, Uwe Kleine-König wrote: > On Thu, Oct 17, 2019 at 01:11:31PM +0200, Thierry Reding wrote: >> On Thu, Oct 17, 2019 at 12:11:16PM +0200, Uwe Kleine-König wrote: >>> On Thu, Oct 17, 2019 at 11:48:08AM +0200, Michal Vokáč wrote: >>>> On 17. 10. 19 10:10, Uwe Kleine-König wrote: >>>>> A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let >>>>> pwm_get_state() return the last implemented state")) changed the >>>>> semantic of pwm_get_state() and disclosed an (as it seems) common >>>>> problem in lowlevel PWM drivers. By not relying on the period and duty >>>>> cycle being retrievable from a disabled PWM this type of problem is >>>>> worked around. >>>>> >>>>> Apart from this issue only calling the pwm_get_state/pwm_apply_state >>>>> combo once is also more effective. >>>>> >>>>> Signed-off-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de >>>>> --- >>>>> Hello, >>>>> >>>>> There are now two reports about 01ccf903edd6 breaking a backlight. As >>>>> far as I understand the problem this is a combination of the backend pwm >>>>> driver yielding surprising results and the pwm-bl driver doing things >>>>> more complicated than necessary. >>>>> >>>>> So I guess this patch works around these problems. Still it would be >>>>> interesting to find out the details in the imx driver that triggers the >>>>> problem. So Adam, can you please instrument the pwm-imx27 driver to >>>>> print *state at the beginning of pwm_imx27_apply() and the end of >>>>> pwm_imx27_get_state() and provide the results? >>>>> >>>>> Note I only compile tested this change. >>>> >>>> Hi Uwe, >>>> I was just about to respond to the "pwm_bl on i.MX6Q broken on 5.4-RC1+" >>>> thread that I have a similar problem when you submitted this patch. >>>> >>>> So here are my few cents: >>>> >>>> My setup is as follows: >>>> - imx6dl-yapp4-draco with i.MX6Solo >>>> - backlight is controlled with inverted PWM signal >>>> - max brightness level = 32, default brightness level set to 32 in DT. >>>> >>>> 1. Almost correct backlight behavior before 01ccf903edd6 ("pwm: Let >>>> pwm_get_state() return the last implemented state): >>>> >>>> - System boots to userspace and backlight is enabled all the time from >>>> power up. >>>> >>>> $ dmesg | grep state >>>> [ 1.763381] get state end: -1811360608, enabled: 0 >>> >>> What is -1811360608? When I wrote "print *state" above, I thought about >>> something like: >>> >>> pr_info("%s: period: %u, duty: %u, polarity: %d, enabled: %d", >>> __func__, state->period, state->duty_cycle, state->polarity, state->enabled); >>> >>> A quick look into drivers/pwm/pwm-imx27.c shows that this is another >>> driver that yields duty_cycle = 0 when the hardware is off. >> >> It seems to me like the best recourse to fix this for now would be to >> patch up the drivers that return 0 when the hardware is off by caching >> the currently configured duty cycle. >> >> How about the patch below? >> >> Thierry >> >> --- >8 --- >> From 15a52a7f1b910804fabd74a5882befd3f9d6bb37 Mon Sep 17 00:00:00 2001 >> From: Thierry Reding thierry.reding@gmail.com >> Date: Thu, 17 Oct 2019 12:56:00 +0200 >> Subject: [PATCH] pwm: imx27: Cache duty cycle register value >> >> The hardware register containing the duty cycle value cannot be accessed >> when the PWM is disabled. This causes the ->get_state() callback to read >> back a duty cycle value of 0, which can confuse consumer drivers. >> >> Signed-off-by: Thierry Reding thierry.reding@gmail.com >> --- >> drivers/pwm/pwm-imx27.c | 31 ++++++++++++++++++++++++------- >> 1 file changed, 24 insertions(+), 7 deletions(-) >> >> diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c >> index ae11d8577f18..4113d5cd4c62 100644 >> --- a/drivers/pwm/pwm-imx27.c >> +++ b/drivers/pwm/pwm-imx27.c >> @@ -85,6 +85,13 @@ struct pwm_imx27_chip { >> struct clk *clk_per; >> void __iomem *mmio_base; >> struct pwm_chip chip; >> + >> + /* >> + * The driver cannot read the current duty cycle from the hardware if >> + * the hardware is disabled. Cache the last programmed duty cycle >> + * value to return in that case. >> + */ >> + unsigned int duty_cycle; >> }; >> #define to_pwm_imx27_chip(chip) container_of(chip, struct pwm_imx27_chip, chip) >> @@ -155,14 +162,17 @@ static void pwm_imx27_get_state(struct pwm_chip *chip, >> tmp = NSEC_PER_SEC * (u64)(period + 2); >> state->period = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk); >> - /* PWMSAR can be read only if PWM is enabled */ >> - if (state->enabled) { >> + /* >> + * PWMSAR can be read only if PWM is enabled. If the PWM is disabled, >> + * use the cached value. >> + */ >> + if (state->enabled) >> val = readl(imx->mmio_base + MX3_PWMSAR); >> - tmp = NSEC_PER_SEC * (u64)(val); >> - state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk); >> - } else { >> - state->duty_cycle = 0; >> - } >> + else >> + val = imx->duty_cycle; >> + >> + tmp = NSEC_PER_SEC * (u64)(val); >> + state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk); >> if (!state->enabled) >> pwm_imx27_clk_disable_unprepare(chip); >> @@ -261,6 +271,13 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm, >> writel(duty_cycles, imx->mmio_base + MX3_PWMSAR); >> writel(period_cycles, imx->mmio_base + MX3_PWMPR); >> + /* >> + * Store the duty cycle for future reference in cases where >> + * the MX3_PWMSAR register can't be read (i.e. when the PWM >> + * is disabled). >> + */ >> + imx->duty_cycle = duty_cycles; >> + > > I wonder if it would be more sensible to do this in the pwm core > instead. Currently there are two drivers known with this problem. I > wouldn't be surprised if there were more.

I've inspected all the drivers and didn't spot any beyond cros-ec and i.MX that have this problem. There's also no good way to do this in the core, because the core doesn't know whether or not the driver is capable of returning the correct duty cycle on hardare readout. So the core would have to rely on state->duty_cycle that is passed in, but then the offending commit becomes useless because the whole point was to return the state as written to hardware (rather than the software state which was being returned before that patch).

> If we want to move clients to not rely on .period and .duty_cycle for a > disabled PWM (do we?) a single change in the core is also beneficial > compared to fixing several lowlevel drivers.

These are really two orthogonal problems. We don't currently consider enabled = 0 to be equivalent to duty_cycle = 0 at an API level. I'm not prepared to do that at this point in the release cycle either.

What this here has shown is that we have at least two drivers that don't behave the way they are supposed to according to the API and they break consumers. If they break for pwm-backlight, it's possible that they will break for other consumers as well. So the right thing to do is fix the two drivers that are broken.

After -rc1 we no longer experiment. Instead we clean up the messes we've made. We can revisit the other points once mainline is fixed.

Hi Thierry, I just tried your patch with v5.4-rc3 with this result:

root@hydraco:~# dmesg | grep pwm_ [ 1.772089] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 4.938759] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 4.947431] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0

Okay... this is interesting. If I understand correctly, that first line here is where the initial hardware readout happens. The second one is the first time when the backlight is configured, so it sets period and polarity. But then for some reason when we read out after that to read what state was written... we see that actually nothing was written at all.

And we can see why in pwm_imx27_apply(): If the PWM is not enabled, we don't actually program any of the registers, so it's not a surprise that things fall apart.

...

[ 4.956484] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 0 [ 4.965473] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 4.974410] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 1 [ 4.988617] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 1

Backlight is on with full brightness at this stage.

root@hydraco:/sys/class/backlight/backlight# cat brightness 32

root@hydraco:/sys/class/backlight/backlight# echo 32 > brightness [ 153.386391] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 1 [ 153.398311] pwm_imx27_get_state: period: 992970, duty: 992970, polarity: 0, enabled: 1

Backlight goes down.

root@hydraco:/sys/class/backlight/backlight# echo 1 > brightness [ 168.506261] pwm_imx27_apply: period: 992970, duty: 15576, polarity: 0, enabled: 1 [ 168.518064] pwm_imx27_get_state: period: 992970, duty: 15576, polarity: 0, enabled: 1

Backlight goes up to almost full brightness.

root@hydraco:/sys/class/backlight/backlight# echo 0 > brightness [ 177.496265] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 177.507602] pwm_imx27_get_state: period: 496485, duty: 7788, polarity: 0, enabled: 0

Backlight goes up to full brightness.

So your patch does not solve my issue.

The main problem I see is incorrect polarity setting. In my DT the pwm-backlight consumer requests PWM_POLARITY_INVERTED and period 500000ns. Though after reset the PWM HW registers are configured to normal polarity. This initial setting is read out and used by the consumer instead of the DT configuration.

So the problem with the i.MX driver is that it doesn't actually write the full state to the hardware and therefore the patch that caused these things to break reads back an incomplete state. So we've basically got two options: 1) make sure the hardware state is fully written or 2) make sure that we return the cached state.

I think 2) doesn't really make sense because it is conflicts with the purpose of the ->get_state() callback. The only time where we should be returning cached data is if the hardware registers don't contain the information (as in the case of the cros-ec driver) or if we can't access it for other reasons (such as in the case of i.MX's duty cycle).

Does the attached patch help with your issue? The idea is to always write the full state to the hardware, even if period and duty cycle are unused when the PWM is disabled. That's really the kind of contract that we have added with the offending patch in the core.

It looks like all other drivers handle this more or less correctly, so if we only need to fix up cros-ec and i.MX this seems like a realistic way to fix things up. If other drivers are problematic in this regard, we should probably revert and then fix the drivers before we can apply that patch again.

This patch combined with your previous patch appears to have worked. If you end up sending a patch series to fix this, go ahead and add

Tested-by: Adam Ford aford173@gmail.com #imx6q-logicpd

Excellent! Thanks for testing this. I'll wait until tomorrow to see if there's some feedback from Enric for the cros-ec change. I'll send out the total of three patches again in the hopes that those are really the only two cases that are broken.

When you do, can you mark it with the Fixes note? I am hoping the maintainers can hopefully incorporate this into 5.4 since it fixes a regression.

Hi Thierry,

I can confirm that the combination of your two patches: - ("pwm: imx27: Unconditionally write state to hardware") - ("pwm: imx27: Cache duty cycle register value")

works OK and solve my problem as well.

root@hydraco:~# dmesg | grep pwm_ [ 1.695306] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 5.387271] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 5.396433] pwm_imx27_get_state: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 5.405500] pwm_imx27_apply: period: 500000, duty: 500000, polarity: 1, enabled: 0 [ 5.418802] pwm_imx27_get_state: period: 500000, duty: 500000, polarity: 1, enabled: 0 [ 5.428208] pwm_imx27_apply: period: 500000, duty: 500000, polarity: 1, enabled: 1 [ 5.442633] pwm_imx27_get_state: period: 500000, duty: 500000, polarity: 1, enabled: 1

Backlight is on from power up to userspace.

root@hydraco:~# cd /sys/class/backlight/backlight/ root@hydraco:/sys/devices/soc0/backlight/backlight/backlight# cat brightness 32

root@hydraco:/sys/devices/soc0/backlight/backlight/backlight# echo 32 > brightness

Nothing happens.

root@hydraco:/sys/devices/soc0/backlight/backlight/backlight# echo 1 > brightness [ 513.629043] pwm_imx27_apply: period: 500000, duty: 7843, polarity: 1, enabled: 1 [ 513.639899] pwm_imx27_get_state: period: 500000, duty: 7833, polarity: 1, enabled: 1

Backlight goes to low brightness.

root@hydraco:/sys/devices/soc0/backlight/backlight/backlight# echo 0 > brightness [ 519.677088] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 519.687733] pwm_imx27_get_state: period: 500000, duty: 0, polarity: 1, enabled: 0

Backlight goes to max brightness, unresolved i.MX6 limitation.

root@hydraco:/sys/devices/soc0/backlight/backlight/backlight# echo 32 > brightness [ 923.921292] pwm_imx27_apply: period: 500000, duty: 500000, polarity: 1, enabled: 0 [ 923.933331] pwm_imx27_get_state: period: 500000, duty: 500000, polarity: 1, enabled: 0 [ 923.944546] pwm_imx27_apply: period: 500000, duty: 500000, polarity: 1, enabled: 1 [ 923.963931] pwm_imx27_get_state: period: 500000, duty: 500000, polarity: 1, enabled: 1

Backlight remains at max brightness, OK.

If I apply the patch from Uwe ("backlight: pwm_bl: configure pwm only once per backlight toggle") on top of that, it still works and I do not see any change in the behavior.

root@hydraco:~# dmesg | grep pwm_ [ 1.687461] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 4.875087] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 4.884796] pwm_imx27_get_state: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 4.893922] pwm_imx27_apply: period: 500000, duty: 500000, polarity: 1, enabled: 1 [ 4.908473] pwm_imx27_get_state: period: 500000, duty: 500000, polarity: 1, enabled: 1

root@hydraco:~# cd /sys/class/backlight/backlight/ root@hydraco:/sys/devices/soc0/backlight/backlight/backlight# cat brightness 32

root@hydraco:/sys/devices/soc0/backlight/backlight/backlight# echo 32 > brightness

root@hydraco:/sys/devices/soc0/backlight/backlight/backlight# echo 1 > brightness [ 110.775650] pwm_imx27_apply: period: 500000, duty: 7843, polarity: 1, enabled: 1 [ 110.786512] pwm_imx27_get_state: period: 500000, duty: 7833, polarity: 1, enabled: 1

root@hydraco:/sys/devices/soc0/backlight/backlight/backlight# echo 0 > brightness [ 128.224036] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 128.234675] pwm_imx27_get_state: period: 500000, duty: 0, polarity: 1, enabled: 0

root@hydraco:/sys/devices/soc0/backlight/backlight/backlight# echo 32 > brightness [ 138.208072] pwm_imx27_apply: period: 500000, duty: 500000, polarity: 1, enabled: 1 [ 138.220271] pwm_imx27_get_state: period: 500000, duty: 500000, polarity: 1, enabled: 1

The only difference is here when the state is changed from enabled=0 to enabled=1. The apply/get_state combo is called only once.

So this looks good to me.

Tested-by: Michal Vokáč michal.vokac@ysoft.com

Thank you all for the very prompt reaction! Michal

...

...

...

--- >8 --- From 7040f0038e04a1caa6dda5b6f675a9fdee0271f4 Mon Sep 17 00:00:00 2001 From: Thierry Reding thierry.reding@gmail.com Date: Thu, 17 Oct 2019 17:11:41 +0200 Subject: [PATCH] pwm: imx27: Unconditionally write state to hardware

The i.MX driver currently uses a shortcut and doesn't write all of the state through to the hardware when the PWM is disabled. This causes an inconsistent state to be read back by consumers with the result of them malfunctioning.

Fix this by always writing the full state through to the hardware registers so that the correct state can always be read back.

Signed-off-by: Thierry Reding thierry.reding@gmail.com

drivers/pwm/pwm-imx27.c | 120 ++++++++++++++++++++-------------------- 1 file changed, 59 insertions(+), 61 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c index 4113d5cd4c62..59d8b1289808 100644 --- a/drivers/pwm/pwm-imx27.c +++ b/drivers/pwm/pwm-imx27.c @@ -230,70 +230,68 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,

     pwm_get_state(pwm, &cstate);

•   if (state->enabled) {
  

•           c = clk_get_rate(imx->clk_per);
  

•           c *= state->period;
  

•           do_div(c, 1000000000);
  

•           period_cycles = c;
  

•           prescale = period_cycles / 0x10000 + 1;
  

•           period_cycles /= prescale;
  

•           c = (unsigned long long)period_cycles * state->duty_cycle;
  

•           do_div(c, state->period);
  

•           duty_cycles = c;
  

•           /*
  

•            * according to imx pwm RM, the real period value should be
  

•            * PERIOD value in PWMPR plus 2.
  

•            */
  

•           if (period_cycles > 2)
  

•                   period_cycles -= 2;
  

•           else
  

•                   period_cycles = 0;
  

•           /*
  

•            * Wait for a free FIFO slot if the PWM is already enabled, and
  

•            * flush the FIFO if the PWM was disabled and is about to be
  

•            * enabled.
  

•            */
  

•           if (cstate.enabled) {
  

•                   pwm_imx27_wait_fifo_slot(chip, pwm);
  

•           } else {
  

•                   ret = pwm_imx27_clk_prepare_enable(chip);
  

•                   if (ret)
  

•                           return ret;
  

•                   pwm_imx27_sw_reset(chip);
  

•           }
  

•           writel(duty_cycles, imx->mmio_base + MX3_PWMSAR);
  

•           writel(period_cycles, imx->mmio_base + MX3_PWMPR);
  

•           /*
  

•            * Store the duty cycle for future reference in cases where
  

•            * the MX3_PWMSAR register can't be read (i.e. when the PWM
  

•            * is disabled).
  

•            */
  

•           imx->duty_cycle = duty_cycles;
  

•           cr = MX3_PWMCR_PRESCALER_SET(prescale) |
  

•                MX3_PWMCR_STOPEN | MX3_PWMCR_DOZEN | MX3_PWMCR_WAITEN |
  

•                FIELD_PREP(MX3_PWMCR_CLKSRC, MX3_PWMCR_CLKSRC_IPG_HIGH) |
  

•                MX3_PWMCR_DBGEN | MX3_PWMCR_EN;
  

•           if (state->polarity == PWM_POLARITY_INVERSED)
  

•                   cr |= FIELD_PREP(MX3_PWMCR_POUTC,
  

•                                   MX3_PWMCR_POUTC_INVERTED);
  

•           writel(cr, imx->mmio_base + MX3_PWMCR);
  

•   } else if (cstate.enabled) {
  

•           writel(0, imx->mmio_base + MX3_PWMCR);
  

•   c = clk_get_rate(imx->clk_per);
  

•   c *= state->period;
  
  

•           pwm_imx27_clk_disable_unprepare(chip);
  

•   do_div(c, 1000000000);
  

•   period_cycles = c;
  

•   prescale = period_cycles / 0x10000 + 1;
  

•   period_cycles /= prescale;
  

•   c = (unsigned long long)period_cycles * state->duty_cycle;
  

•   do_div(c, state->period);
  

•   duty_cycles = c;
  

•   /*
  

•    * according to imx pwm RM, the real period value should be PERIOD
  

•    * value in PWMPR plus 2.
  

•    */
  

•   if (period_cycles > 2)
  

•           period_cycles -= 2;
  

•   else
  

•           period_cycles = 0;
  

•   /*
  

•    * Wait for a free FIFO slot if the PWM is already enabled, and flush
  

•    * the FIFO if the PWM was disabled and is about to be enabled.
  

•    */
  

•   if (cstate.enabled) {
  

•           pwm_imx27_wait_fifo_slot(chip, pwm);
  

•   } else {
  

•           ret = pwm_imx27_clk_prepare_enable(chip);
  

•           if (ret)
  

•                   return ret;
  

•           pwm_imx27_sw_reset(chip);
     }
  
  

•   writel(duty_cycles, imx->mmio_base + MX3_PWMSAR);
  

•   writel(period_cycles, imx->mmio_base + MX3_PWMPR);
  

•   /*
  

•    * Store the duty cycle for future reference in cases where the
  

•    * MX3_PWMSAR register can't be read (i.e. when the PWM is disabled).
  

•    */
  

•   imx->duty_cycle = duty_cycles;
  

•   cr = MX3_PWMCR_PRESCALER_SET(prescale) |
  

•        MX3_PWMCR_STOPEN | MX3_PWMCR_DOZEN | MX3_PWMCR_WAITEN |
  

•        FIELD_PREP(MX3_PWMCR_CLKSRC, MX3_PWMCR_CLKSRC_IPG_HIGH) |
  

•        MX3_PWMCR_DBGEN;
  

•   if (state->polarity == PWM_POLARITY_INVERSED)
  

•           cr |= FIELD_PREP(MX3_PWMCR_POUTC,
  

•                           MX3_PWMCR_POUTC_INVERTED);
  

•   if (state->enabled)
  

•           cr |= MX3_PWMCR_EN;
  

•   writel(cr, imx->mmio_base + MX3_PWMCR);
  

•   if (!state->enabled && cstate.enabled)
  

•           pwm_imx27_clk_disable_unprepare(chip);
  

•    return 0;
  

  }

-- 2.23.0

Hello Thierry,

On Thu, Oct 17, 2019 at 05:14:37PM +0200, Thierry Reding wrote:

On Thu, Oct 17, 2019 at 03:58:25PM +0200, Michal Vokáč wrote:

...

On 17. 10. 19 14:59, Thierry Reding wrote:

...

On Thu, Oct 17, 2019 at 02:09:17PM +0200, Uwe Kleine-König wrote:

...

On Thu, Oct 17, 2019 at 01:11:31PM +0200, Thierry Reding wrote:

...

On Thu, Oct 17, 2019 at 12:11:16PM +0200, Uwe Kleine-König wrote:

...

On Thu, Oct 17, 2019 at 11:48:08AM +0200, Michal Vokáč wrote: > On 17. 10. 19 10:10, Uwe Kleine-König wrote: > > A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let > > pwm_get_state() return the last implemented state")) changed the > > semantic of pwm_get_state() and disclosed an (as it seems) common > > problem in lowlevel PWM drivers. By not relying on the period and duty > > cycle being retrievable from a disabled PWM this type of problem is > > worked around. > > > > Apart from this issue only calling the pwm_get_state/pwm_apply_state > > combo once is also more effective. > > > > Signed-off-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de > > --- > > Hello, > > > > There are now two reports about 01ccf903edd6 breaking a backlight. As > > far as I understand the problem this is a combination of the backend pwm > > driver yielding surprising results and the pwm-bl driver doing things > > more complicated than necessary. > > > > So I guess this patch works around these problems. Still it would be > > interesting to find out the details in the imx driver that triggers the > > problem. So Adam, can you please instrument the pwm-imx27 driver to > > print *state at the beginning of pwm_imx27_apply() and the end of > > pwm_imx27_get_state() and provide the results? > > > > Note I only compile tested this change. > > Hi Uwe, > I was just about to respond to the "pwm_bl on i.MX6Q broken on 5.4-RC1+" > thread that I have a similar problem when you submitted this patch. > > So here are my few cents: > > My setup is as follows: > - imx6dl-yapp4-draco with i.MX6Solo > - backlight is controlled with inverted PWM signal > - max brightness level = 32, default brightness level set to 32 in DT. > > 1. Almost correct backlight behavior before 01ccf903edd6 ("pwm: Let > pwm_get_state() return the last implemented state): > > - System boots to userspace and backlight is enabled all the time from > power up. > > $ dmesg | grep state > [ 1.763381] get state end: -1811360608, enabled: 0

What is -1811360608? When I wrote "print *state" above, I thought about something like:

pr_info("%s: period: %u, duty: %u, polarity: %d, enabled: %d", __func__, state->period, state->duty_cycle, state->polarity, state->enabled);

A quick look into drivers/pwm/pwm-imx27.c shows that this is another driver that yields duty_cycle = 0 when the hardware is off.

It seems to me like the best recourse to fix this for now would be to patch up the drivers that return 0 when the hardware is off by caching the currently configured duty cycle.

How about the patch below?

Thierry

--- >8 --- From 15a52a7f1b910804fabd74a5882befd3f9d6bb37 Mon Sep 17 00:00:00 2001 From: Thierry Reding thierry.reding@gmail.com Date: Thu, 17 Oct 2019 12:56:00 +0200 Subject: [PATCH] pwm: imx27: Cache duty cycle register value

The hardware register containing the duty cycle value cannot be accessed when the PWM is disabled. This causes the ->get_state() callback to read back a duty cycle value of 0, which can confuse consumer drivers.

Signed-off-by: Thierry Reding thierry.reding@gmail.com

drivers/pwm/pwm-imx27.c | 31 ++++++++++++++++++++++++------- 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c index ae11d8577f18..4113d5cd4c62 100644 --- a/drivers/pwm/pwm-imx27.c +++ b/drivers/pwm/pwm-imx27.c @@ -85,6 +85,13 @@ struct pwm_imx27_chip { struct clk *clk_per; void __iomem *mmio_base; struct pwm_chip chip;

• /*

• * The driver cannot read the current duty cycle from the hardware if
  

• * the hardware is disabled. Cache the last programmed duty cycle
  

• * value to return in that case.
  

• */
  

• unsigned int duty_cycle; }; #define to_pwm_imx27_chip(chip) container_of(chip, struct pwm_imx27_chip, chip)

@@ -155,14 +162,17 @@ static void pwm_imx27_get_state(struct pwm_chip *chip, tmp = NSEC_PER_SEC * (u64)(period + 2); state->period = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);

• /* PWMSAR can be read only if PWM is enabled */
• if (state->enabled) {

• /*

• * PWMSAR can be read only if PWM is enabled. If the PWM is disabled,
  

• * use the cached value.
  

• */
  

• if (state->enabled) val = readl(imx->mmio_base + MX3_PWMSAR);

• tmp = NSEC_PER_SEC * (u64)(val);
  

• state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);
  

• } else {

• state->duty_cycle = 0;
  

• }

• else

• val = imx->duty_cycle;
  

• tmp = NSEC_PER_SEC * (u64)(val);
• state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk); if (!state->enabled) pwm_imx27_clk_disable_unprepare(chip);

@@ -261,6 +271,13 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm, writel(duty_cycles, imx->mmio_base + MX3_PWMSAR); writel(period_cycles, imx->mmio_base + MX3_PWMPR);

• /*
  

•  * Store the duty cycle for future reference in cases where
  

•  * the MX3_PWMSAR register can't be read (i.e. when the PWM
  

•  * is disabled).
  

•  */
  

• imx->duty_cycle = duty_cycles;
  

I wonder if it would be more sensible to do this in the pwm core instead. Currently there are two drivers known with this problem. I wouldn't be surprised if there were more.

I've inspected all the drivers and didn't spot any beyond cros-ec and i.MX that have this problem. There's also no good way to do this in the core, because the core doesn't know whether or not the driver is capable of returning the correct duty cycle on hardare readout. So the core would have to rely on state->duty_cycle that is passed in, but then the offending commit becomes useless because the whole point was to return the state as written to hardware (rather than the software state which was being returned before that patch).

...

If we want to move clients to not rely on .period and .duty_cycle for a disabled PWM (do we?) a single change in the core is also beneficial compared to fixing several lowlevel drivers.

These are really two orthogonal problems. We don't currently consider enabled = 0 to be equivalent to duty_cycle = 0 at an API level. I'm not prepared to do that at this point in the release cycle either.

What this here has shown is that we have at least two drivers that don't behave the way they are supposed to according to the API and they break consumers. If they break for pwm-backlight, it's possible that they will break for other consumers as well. So the right thing to do is fix the two drivers that are broken.

After -rc1 we no longer experiment. Instead we clean up the messes we've made. We can revisit the other points once mainline is fixed.

Hi Thierry, I just tried your patch with v5.4-rc3 with this result:

root@hydraco:~# dmesg | grep pwm_ [ 1.772089] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 4.938759] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 4.947431] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0

Okay... this is interesting. If I understand correctly, that first line here is where the initial hardware readout happens. The second one is the first time when the backlight is configured, so it sets period and polarity. But then for some reason when we read out after that to read what state was written... we see that actually nothing was written at all.

And we can see why in pwm_imx27_apply(): If the PWM is not enabled, we don't actually program any of the registers, so it's not a surprise that things fall apart.

...

[ 4.956484] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 0 [ 4.965473] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 4.974410] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 1 [ 4.988617] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 1

Backlight is on with full brightness at this stage.

root@hydraco:/sys/class/backlight/backlight# cat brightness 32

root@hydraco:/sys/class/backlight/backlight# echo 32 > brightness [ 153.386391] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 1 [ 153.398311] pwm_imx27_get_state: period: 992970, duty: 992970, polarity: 0, enabled: 1

Backlight goes down.

root@hydraco:/sys/class/backlight/backlight# echo 1 > brightness [ 168.506261] pwm_imx27_apply: period: 992970, duty: 15576, polarity: 0, enabled: 1 [ 168.518064] pwm_imx27_get_state: period: 992970, duty: 15576, polarity: 0, enabled: 1

Backlight goes up to almost full brightness.

root@hydraco:/sys/class/backlight/backlight# echo 0 > brightness [ 177.496265] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 177.507602] pwm_imx27_get_state: period: 496485, duty: 7788, polarity: 0, enabled: 0

Backlight goes up to full brightness.

So your patch does not solve my issue.

The main problem I see is incorrect polarity setting. In my DT the pwm-backlight consumer requests PWM_POLARITY_INVERTED and period 500000ns. Though after reset the PWM HW registers are configured to normal polarity. This initial setting is read out and used by the consumer instead of the DT configuration.

So the problem with the i.MX driver is that it doesn't actually write the full state to the hardware and therefore the patch that caused these things to break reads back an incomplete state. So we've basically got two options: 1) make sure the hardware state is fully written or 2) make sure that we return the cached state.

My preference would be 3) make consumers not rely on .duty_cycle and .period if the PWM is disabled. This questions of course the common PWM idiom

pwm_get_state(...) only = change(what, you, think, is, necessary); pwm_apply_state(...)

but I don't like this that much anyhow. YMMV.

Best regards Uwe

On Thu, Oct 17, 2019 at 6:11 AM Thierry Reding thierry.reding@gmail.com wrote:

On Thu, Oct 17, 2019 at 12:11:16PM +0200, Uwe Kleine-König wrote:

...

On Thu, Oct 17, 2019 at 11:48:08AM +0200, Michal Vokáč wrote:

...

On 17. 10. 19 10:10, Uwe Kleine-König wrote:

...

A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state")) changed the semantic of pwm_get_state() and disclosed an (as it seems) common problem in lowlevel PWM drivers. By not relying on the period and duty cycle being retrievable from a disabled PWM this type of problem is worked around.

Apart from this issue only calling the pwm_get_state/pwm_apply_state combo once is also more effective.

Signed-off-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de

Hello,

There are now two reports about 01ccf903edd6 breaking a backlight. As far as I understand the problem this is a combination of the backend pwm driver yielding surprising results and the pwm-bl driver doing things more complicated than necessary.

So I guess this patch works around these problems. Still it would be interesting to find out the details in the imx driver that triggers the problem. So Adam, can you please instrument the pwm-imx27 driver to print *state at the beginning of pwm_imx27_apply() and the end of pwm_imx27_get_state() and provide the results?

Note I only compile tested this change.

Hi Uwe, I was just about to respond to the "pwm_bl on i.MX6Q broken on 5.4-RC1+" thread that I have a similar problem when you submitted this patch.

So here are my few cents:

My setup is as follows:

• imx6dl-yapp4-draco with i.MX6Solo
• backlight is controlled with inverted PWM signal
• max brightness level = 32, default brightness level set to 32 in DT.

1. Almost correct backlight behavior before 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state):

• System boots to userspace and backlight is enabled all the time from power up.

  $ dmesg | grep state [ 1.763381] get state end: -1811360608, enabled: 0

What is -1811360608? When I wrote "print *state" above, I thought about something like:

  pr_info("%s: period: %u, duty: %u, polarity: %d, enabled: %d",
          __func__, state->period, state->duty_cycle, state->polarity, state->enabled);

A quick look into drivers/pwm/pwm-imx27.c shows that this is another driver that yields duty_cycle = 0 when the hardware is off.

It seems to me like the best recourse to fix this for now would be to patch up the drivers that return 0 when the hardware is off by caching the currently configured duty cycle.

How about the patch below?

Thierry

--- >8 --- From 15a52a7f1b910804fabd74a5882befd3f9d6bb37 Mon Sep 17 00:00:00 2001 From: Thierry Reding thierry.reding@gmail.com Date: Thu, 17 Oct 2019 12:56:00 +0200 Subject: [PATCH] pwm: imx27: Cache duty cycle register value

The hardware register containing the duty cycle value cannot be accessed when the PWM is disabled. This causes the ->get_state() callback to read back a duty cycle value of 0, which can confuse consumer drivers.

Signed-off-by: Thierry Reding thierry.reding@gmail.com

drivers/pwm/pwm-imx27.c | 31 ++++++++++++++++++++++++------- 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c index ae11d8577f18..4113d5cd4c62 100644 --- a/drivers/pwm/pwm-imx27.c +++ b/drivers/pwm/pwm-imx27.c @@ -85,6 +85,13 @@ struct pwm_imx27_chip { struct clk *clk_per; void __iomem *mmio_base; struct pwm_chip chip;

•   /*
  

•    * The driver cannot read the current duty cycle from the hardware if
  

•    * the hardware is disabled. Cache the last programmed duty cycle
  

•    * value to return in that case.
  

•    */
  

•   unsigned int duty_cycle;
  

};

#define to_pwm_imx27_chip(chip) container_of(chip, struct pwm_imx27_chip, chip) @@ -155,14 +162,17 @@ static void pwm_imx27_get_state(struct pwm_chip *chip, tmp = NSEC_PER_SEC * (u64)(period + 2); state->period = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);

•   /* PWMSAR can be read only if PWM is enabled */
  

•   if (state->enabled) {
  

•   /*
  

•    * PWMSAR can be read only if PWM is enabled. If the PWM is disabled,
  

•    * use the cached value.
  

•    */
  

•   if (state->enabled)
            val = readl(imx->mmio_base + MX3_PWMSAR);
  

•           tmp = NSEC_PER_SEC * (u64)(val);
  

•           state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);
  

•   } else {
  

•           state->duty_cycle = 0;
  

•   }
  

•   else
  

•           val = imx->duty_cycle;
  

•   tmp = NSEC_PER_SEC * (u64)(val);
  

•   state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);
  

Is this right? It seems like the tmp and state->duty_cycle caltulations should be kept inside "if (state->enabled)" because if we set val to the duty_cycle in the else, I would think it is going to calculate this again.

I think the 'else' should be 'state->duty_cycle = imx->duty_cycle' because we shouldn't need to recalculate this again.

Am I missing something?

adam

    if (!state->enabled)
            pwm_imx27_clk_disable_unprepare(chip);

@@ -261,6 +271,13 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm, writel(duty_cycles, imx->mmio_base + MX3_PWMSAR); writel(period_cycles, imx->mmio_base + MX3_PWMPR);

•           /*
  

•            * Store the duty cycle for future reference in cases where
  

•            * the MX3_PWMSAR register can't be read (i.e. when the PWM
  

•            * is disabled).
  

•            */
  

•           imx->duty_cycle = duty_cycles;
  

•           cr = MX3_PWMCR_PRESCALER_SET(prescale) |
                 MX3_PWMCR_STOPEN | MX3_PWMCR_DOZEN | MX3_PWMCR_WAITEN |
                 FIELD_PREP(MX3_PWMCR_CLKSRC, MX3_PWMCR_CLKSRC_IPG_HIGH) |
  

-- 2.23.0

On 17. 10. 19 11:48, Michal Vokáč wrote:

On 17. 10. 19 10:10, Uwe Kleine-König wrote:

...

A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state")) changed the semantic of pwm_get_state() and disclosed an (as it seems) common problem in lowlevel PWM drivers. By not relying on the period and duty cycle being retrievable from a disabled PWM this type of problem is worked around.

Apart from this issue only calling the pwm_get_state/pwm_apply_state combo once is also more effective.

Signed-off-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de

Hello,

There are now two reports about 01ccf903edd6 breaking a backlight. As far as I understand the problem this is a combination of the backend pwm driver yielding surprising results and the pwm-bl driver doing things more complicated than necessary.

So I guess this patch works around these problems. Still it would be interesting to find out the details in the imx driver that triggers the problem. So Adam, can you please instrument the pwm-imx27 driver to print *state at the beginning of pwm_imx27_apply() and the end of pwm_imx27_get_state() and provide the results?

Note I only compile tested this change.

Hi Uwe, I was just about to respond to the "pwm_bl on i.MX6Q broken on 5.4-RC1+" thread that I have a similar problem when you submitted this patch.

So here are my few cents:

Once again with updated and more detailed debug messages.

My setup is as follows:

• imx6dl-yapp4-draco with i.MX6Solo
• backlight is controlled with inverted PWM signal
• max brightness level = 32, default brightness level set to 32 in DT.

1. Almost correct backlight behavior before 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state):

• System boots to userspace and backlight is enabled all the time from power up.

- $ dmesg | grep pwm_ [ 1.761546] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 5.012352] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 5.021143] pwm_imx27_apply: period: 500000, duty: 500000, polarity: 1, enabled: 0 [ 5.030182] pwm_imx27_apply: period: 500000, duty: 500000, polarity: 1, enabled: 1

• $ cat brightness 32

• $ echo 32 > brightness # nothing happens, max. brightness

• $ echo 1 > brightness # backlight goes down to lowest level

[ 93.976354] pwm_imx27_apply: period: 500000, duty: 7843, polarity: 1, enabled: 1

• $ echo 0 > brightness # backlight goes up to max. level, this is # problem of the inverted PWM on i.MX we attempted # to solve some time ago.

[ 115.496350] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0

2. Backlight behavior on v5.4-rc3:

• System boots to userspace and backlight is enabled all the time from power up.

- $ dmesg | grep pwm_ [ 1.774071] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 5.003961] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 5.012649] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 5.021694] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 0 [ 5.030732] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 5.039643] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 1 [ 5.049605] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 1

• $ cat brightness 32

• $ echo 32 > brightness # backlight goes down

[ 707.946970] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 1 [ 707.958551] pwm_imx27_get_state: period: 992970, duty: 992970, polarity: 0, enabled: 1

• $ echo 1 > brightness # backlight goes up to high level

[ 757.516845] pwm_imx27_apply: period: 992970, duty: 15576, polarity: 0, enabled: 1 [ 757.528438] pwm_imx27_get_state: period: 992970, duty: 15576, polarity: 0, enabled: 1

• $ echo 0 > brightness # backlight goes up to highest level

[ 783.386838] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 783.398025] pwm_imx27_get_state: period: 496485, duty: 0, polarity: 0, enabled: 0

So the behavior is clearly inverted to how it worked prior to 01ccf903edd6 with the weird exception that the initial brightness level 32 is not applied.

3. Backlight behavior on v5.4-rc3 + this patch:

• System boots with backlight enabled. In the middle of kernel boot backlight is disabled.

• $ dmesg | grep state

[ 1.773099] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 5.002532] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0 [ 5.011263] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 5.020307] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 1 [ 5.031066] pwm_imx27_get_state: period: 992970, duty: 992970, polarity: 0, enabled: 1

• $ cat brightness 32

• $ echo 32 > brightness # nothing happens, backlight is down

• $ echo 1 > brightness # backlight goes to high level

[ 73.786926] pwm_imx27_apply: period: 992970, duty: 15576, polarity: 0, enabled: 1 [ 73.798469] pwm_imx27_get_state: period: 992970, duty: 15576, polarity: 0, enabled: 1

• $ echo 0 > brightness # backlight goes to max brightness

[ 104.636908] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 0 [ 104.648093] pwm_imx27_get_state: period: 496485, duty: 0, polarity: 0, enabled: 0

Same behavior as (2) but the default state from DT is apparently applied.

I only did this experiments. I did not delve into the code to track what is going on in there yet.

Hopefully this helps you a bit and feel free to request other experiments, Michal

...

drivers/video/backlight/pwm_bl.c | 34 +++++++++++--------------------- 1 file changed, 12 insertions(+), 22 deletions(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c index 746eebc411df..ddebd62b3978 100644 --- a/drivers/video/backlight/pwm_bl.c +++ b/drivers/video/backlight/pwm_bl.c @@ -42,10 +42,8 @@ struct pwm_bl_data { static void pwm_backlight_power_on(struct pwm_bl_data *pb) {

• struct pwm_state state; int err;
• pwm_get_state(pb->pwm, &state); if (pb->enabled) return;

@@ -53,9 +51,6 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb) if (err < 0) dev_err(pb->dev, "failed to enable power supply\n");

• state.enabled = true;
• pwm_apply_state(pb->pwm, &state);

• if (pb->post_pwm_on_delay)
      msleep(pb->post_pwm_on_delay);
  

@@ -67,40 +62,27 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb) static void pwm_backlight_power_off(struct pwm_bl_data *pb) {

• struct pwm_state state;
• pwm_get_state(pb->pwm, &state);
• if (!pb->enabled)

•    return;
  

• if (pb->enable_gpio)
      gpiod_set_value_cansleep(pb->enable_gpio, 0);
  if (pb->pwm_off_delay)
      msleep(pb->pwm_off_delay);
  

• state.enabled = false;
• state.duty_cycle = 0;
• pwm_apply_state(pb->pwm, &state);

• regulator_disable(pb->power_supply);
  pb->enabled = false;
  

  }

-static int compute_duty_cycle(struct pwm_bl_data *pb, int brightness) +static int compute_duty_cycle(struct pwm_bl_data *pb, int brightness, struct pwm_state *state) { unsigned int lth = pb->lth_brightness;

• struct pwm_state state; u64 duty_cycle;
• pwm_get_state(pb->pwm, &state);

• if (pb->levels)
      duty_cycle = pb->levels[brightness];
  else
      duty_cycle = brightness;
  

• duty_cycle *= state.period - lth;

• duty_cycle *= state->period - lth; do_div(duty_cycle, pb->scale); return duty_cycle + lth;

@@ -122,12 +104,20 @@ static int pwm_backlight_update_status(struct backlight_device *bl) if (brightness > 0) { pwm_get_state(pb->pwm, &state);

•    state.duty_cycle = compute_duty_cycle(pb, brightness);
  

•    state.duty_cycle = compute_duty_cycle(pb, brightness, &state);
  

•    state.enabled = true;
      pwm_apply_state(pb->pwm, &state);
  

•     pwm_backlight_power_on(pb);
  

• } else

• } else { pwm_backlight_power_off(pb);

•    pwm_get_state(pb->pwm, &state);
  

•    state.enabled = false;
  

•    state.duty_cycle = 0;
  

•    pwm_apply_state(pb->pwm, &state);
  

• }

• if (pb->notify_after)
      pb->notify_after(pb->dev, brightness);
  
  

On Thu, Oct 17, 2019 at 7:19 AM Uwe Kleine-König u.kleine-koenig@pengutronix.de wrote:

On Thu, Oct 17, 2019 at 12:47:27PM +0100, Daniel Thompson wrote:

...

On Thu, Oct 17, 2019 at 10:10:59AM +0200, Uwe Kleine-König wrote:

...

A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state")) changed the semantic of pwm_get_state() and disclosed an (as it seems) common problem in lowlevel PWM drivers. By not relying on the period and duty cycle being retrievable from a disabled PWM this type of problem is worked around.

Apart from this issue only calling the pwm_get_state/pwm_apply_state combo once is also more effective.

I'm only interested in the second paragraph here.

There seems to be a reasonable consensus that the i.MX27 and cros-ec PWM drivers should be fixed for the benefit of other PWM clients. So we make this change because it makes the pwm-bl better... not to work around bugs ;-).

That's fine, still I think it's fair to explain the motivation of creating this patch.

...

...

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c index 746eebc411df..ddebd62b3978 100644 --- a/drivers/video/backlight/pwm_bl.c +++ b/drivers/video/backlight/pwm_bl.c @@ -67,40 +62,27 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb)

static void pwm_backlight_power_off(struct pwm_bl_data *pb) {

• struct pwm_state state;
• pwm_get_state(pb->pwm, &state);
• if (!pb->enabled)

•       return;
  

Why remove the pb->enabled check? I thought that was there to ensure we don't mess up the regular reference counts.

I havn't looked yet, but I guess I have to respin. Expect a v2 later today.

I would agree that a high-level fix is better than a series of low level driver fixes. For what its worth, your V1 patch worked fine on my i.MX6Q. I can test the V2 patch when its ready.

adam

Best regards Uwe

-- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ |

On Thu, Oct 17, 2019 at 07:40:47AM -0500, Adam Ford wrote:

On Thu, Oct 17, 2019 at 7:34 AM Adam Ford aford173@gmail.com wrote:

...

On Thu, Oct 17, 2019 at 7:19 AM Uwe Kleine-König u.kleine-koenig@pengutronix.de wrote:

...

On Thu, Oct 17, 2019 at 12:47:27PM +0100, Daniel Thompson wrote:

...

On Thu, Oct 17, 2019 at 10:10:59AM +0200, Uwe Kleine-König wrote:

...

A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let pwm_get_state() return the last implemented state")) changed the semantic of pwm_get_state() and disclosed an (as it seems) common problem in lowlevel PWM drivers. By not relying on the period and duty cycle being retrievable from a disabled PWM this type of problem is worked around.

Apart from this issue only calling the pwm_get_state/pwm_apply_state combo once is also more effective.

I'm only interested in the second paragraph here.

There seems to be a reasonable consensus that the i.MX27 and cros-ec PWM drivers should be fixed for the benefit of other PWM clients. So we make this change because it makes the pwm-bl better... not to work around bugs ;-).

That's fine, still I think it's fair to explain the motivation of creating this patch.

...

...

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c index 746eebc411df..ddebd62b3978 100644 --- a/drivers/video/backlight/pwm_bl.c +++ b/drivers/video/backlight/pwm_bl.c @@ -67,40 +62,27 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb)

static void pwm_backlight_power_off(struct pwm_bl_data *pb) {

• struct pwm_state state;
• pwm_get_state(pb->pwm, &state);
• if (!pb->enabled)

•       return;
  

Why remove the pb->enabled check? I thought that was there to ensure we don't mess up the regular reference counts.

I havn't looked yet, but I guess I have to respin. Expect a v2 later today.

I would agree that a high-level fix is better than a series of low level driver fixes. For what its worth, your V1 patch worked fine on my i.MX6Q. I can test the V2 patch when its ready.

I may have spoken too soon. The patch fixes the display in that it comes on when it previously did not, but changes to brightness do not appear to do anything anymore. I don't have an oscilloscope where I am now, so I cannot verify whether or not the PWM duty cycle changes.

To my eye, the following do not appear to change the brightness of the screen: echo 7 > /sys/devices/soc0/backlight-lvds/backlight/backlight-lvds/brightness echo 2 > /sys/devices/soc0/backlight-lvds/backlight/backlight-lvds/brightness echo 5 > /sys/devices/soc0/backlight-lvds/backlight/backlight-lvds/brightness

Hi Adam,

can you try the i.MX PWM patch that I posted earlier instead? I really think we need to fix this in the PWM drivers because they are broken. pwm-backlight is not. -rc3 is really not a time to work around breakage in consumers.

If my patch doesn't help, can you try reverting the offending patch? If we can't come up with a good fix for the drivers, reverting is the next best option.

Thierry