On Wed 23 Jun 18:37 CDT 2021, Doug Anderson wrote:
Hi,
On Tue, Jun 22, 2021 at 8:28 PM Bjorn Andersson bjorn.andersson@linaro.org wrote:
+static int ti_sn65dsi86_read_u16(struct ti_sn65dsi86 *pdata,
unsigned int reg, u16 *val)+{
unsigned int tmp;int ret;ret = regmap_read(pdata->regmap, reg, &tmp);if (ret)return ret;*val = tmp;ret = regmap_read(pdata->regmap, reg + 1, &tmp);if (ret)return ret;*val |= tmp << 8;return 0;+}
static void ti_sn65dsi86_write_u16(struct ti_sn65dsi86 *pdata, unsigned int reg, u16 val)
I suspect we might want to update this function to use regmap_bulk_write(). I believe that will allow PWM updates to happen in a single i2c transaction. I don't know whether the bridge chip implements that, but conceivably it could use this information to avoid discontinuities when updating the "high" and "low" parts of a register. Even if the bridge chip doesn't do anything special, though, it will reduce the amount of time that they are inconsistent because it'll be a single transaction on the bus rather than two separate ones.
You bulk_write in ti_sn_aux_transfer() so I don't see why it wouldn't work here, I'll update this - even though I don't know if it will actually result in the two bytes being updated "atomically".
{ @@ -253,6 +297,14 @@ static void ti_sn_bridge_set_refclk_freq(struct ti_sn65dsi86 *pdata)
regmap_update_bits(pdata->regmap, SN_DPPLL_SRC_REG, REFCLK_FREQ_MASK, REFCLK_FREQ(i));
+#if defined(CONFIG_PWM)
/** The PWM refclk is based on the value written to SN_DPPLL_SRC_REG,* regardless of its actual sourcing.*/pdata->pwm_refclk_freq = ti_sn_bridge_refclk_lut[i];+#endif
I really dislike #ifdefs inline in functions. Personally I'd rather you just always put the member in the structure regardless of CONFIG_PWM and always set it.
I'd be happy to do so.
+/*
- Limitations:
- The PWM signal is not driven when the chip is powered down, or in its
- reset state and the driver does not implement the "suspend state"
- described in the documentation. In order to save power, state->enabled is
- interpreted as denoting if the signal is expected to be valid, and is used to keep
- the determine if the chip needs to be kept powered.
"and is used to keep the determine" ? Something about that wording doesn't make sense to me.
Let's subtract "keep the" from that sentence...
- Changing both period and duty_cycle is not done atomically, so the output
- might briefly be a mix of the two settings.
In fact there's nothing atomic about _any_ of the updates, right? We're setting the high and low bytes in separate transactions so if you were watching carefully you might see this if you bumped the PWM up by 1:
0x03ff 0x04ff 0x0400
I've tested this several times with
for i in `seq 4095`; do echo $i > /sys/class/backlight/backlight/brightness; done
And I am not able to see that "transient", but let's improve the u16 write anyways.
- */
+static int ti_sn_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
const struct pwm_state *state)+{
struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);unsigned int pwm_en_inv;unsigned int backlight;unsigned int pre_div;unsigned int scale;u64 period_max;u64 actual;u64 period;int ret;if (!pdata->pwm_enabled) {ret = pm_runtime_get_sync(pdata->dev);if (ret < 0)return ret;You hit the classic pm_runtime trap! :-) You must always call put even if get fails. I think a "goto out" would do it?
Another one bits the dust...
Thanks for teaching me.
}if (state->enabled) {if (!pdata->pwm_enabled) {/** The chip might have been powered down while we* didn't hold a PM runtime reference, so mux in the* PWM function on the GPIO pin again.*/ret = regmap_update_bits(pdata->regmap, SN_GPIO_CTRL_REG,SN_GPIO_MUX_MASK << (2 * SN_PWM_GPIO_IDX),SN_GPIO_MUX_SPECIAL << (2 * SN_PWM_GPIO_IDX));if (ret) {dev_err(pdata->dev, "failed to mux in PWM function\n");goto out;}}/** Per the datasheet the PWM frequency is given by:** PWM_FREQ = REFCLK_FREQ / (PWM_PRE_DIV * BACKLIGHT_SCALE + 1)** which can be rewritten:** T_pwm * REFCLK_FREQ - 1 = PWM_PRE_DIV * BACKLIGHT_SCALE** In order to keep BACKLIGHT_SCALE within its 16 bits,* PWM_PRE_DIV must be:** PWM_PRE_DIV >= (T_pwm * REFCLK_FREQ - 1) / BACKLIGHT_SCALE_MAX;** To simplify the search and optimize the resolution of the* PWM, the lowest possible PWM_PRE_DIV is used. Finally the* scale is calculated as:** BACKLIGHT_SCALE = (T_pwm * REFCLK_FREQ - 1) / PWM_PRE_DIV** Here T_pwm is represented in seconds, so appropriate scaling* to nanoseconds is necessary.*//* Minimum T_pwm is (0 * 0 + 1) / REFCLK_FREQ */if (state->period <= NSEC_PER_SEC / pdata->pwm_refclk_freq) {ret = -EINVAL;goto out;}/** Maximum T_pwm is (255 * 65535 + 1) / * REFCLK_FREQ* Limit period to this to avoid overflows*/period_max = div_u64((u64)NSEC_PER_SEC * (255 * 65535 + 1), pdata->pwm_refclk_freq);if (period > period_max)period = period_max;elseperiod = state->period;pre_div = DIV64_U64_ROUND_UP((period * pdata->pwm_refclk_freq - NSEC_PER_SEC),((u64)NSEC_PER_SEC * BACKLIGHT_SCALE_MAX));scale = div64_u64(period * pdata->pwm_refclk_freq - NSEC_PER_SEC,(u64)NSEC_PER_SEC * pre_div);/** The documentation has the duty ratio given as:** duty BACKLIGHT* ------- = ---------------------* period BACKLIGHT_SCALE + 1** Solve for BACKLIGHT gives us:*/actual = DIV_ROUND_UP_ULL((u64)NSEC_PER_SEC * (pre_div * scale + 1),pdata->pwm_refclk_freq);backlight = div64_u64(state->duty_cycle * (scale + 1), actual);if (backlight > scale)backlight = scale;ret = regmap_write(pdata->regmap, SN_PWM_PRE_DIV_REG, pre_div);if (ret) {dev_err(pdata->dev, "failed to update PWM_PRE_DIV\n");goto out;}ti_sn65dsi86_write_u16(pdata, SN_BACKLIGHT_SCALE_REG, scale);ti_sn65dsi86_write_u16(pdata, SN_BACKLIGHT_REG, backlight);}pwm_en_inv = FIELD_PREP(SN_PWM_EN_MASK, !!state->enabled) |nit: no need for "!!". state->enabled is a boolean.
Ahh, you're right.
FIELD_PREP(SN_PWM_INV_MASK, state->polarity == PWM_POLARITY_INVERSED);ret = regmap_write(pdata->regmap, SN_PWM_EN_INV_REG, pwm_en_inv);if (ret) {dev_err(pdata->dev, "failed to update PWM_EN/PWM_INV\n");goto out;}pdata->pwm_enabled = !!state->enabled;nit: no need for "!!". state->enabled is a boolean.
+out:
if (!pdata->pwm_enabled)pm_runtime_put_sync(pdata->dev);return ret;+}
note: I didn't look at _any_ of your logic here. I figure that you and Uwe already broke your brains on it. I'll try to take a quick peek once you guys come to come agreement.
I think we're approaching a conclusion, at which time I certainly appreciate the additional review :)
One note: in theory it ought to be not impossible to measure this even if you're not an EE if you happen to have access to something like a Salae Logic 16. The PWM ought to go out on the cable connecting to the LCD on one of the pins and those pins tend to be easy enough to probe that even a noob like myself can probe them. Of course it does mean opening up your device...
If I had access to the signal I would have already thrown my oscilloscope at it and answered the question. But I only have this chip inside my production-Lenovo Yoga C630...
+static int ti_sn_bridge_gpio_request(struct gpio_chip *chip, unsigned int offset) +{
struct ti_sn65dsi86 *pdata = gpiochip_get_data(chip);if (offset == SN_PWM_GPIO_IDX)return ti_sn_pwm_pin_request(pdata);return 0;+}
static void ti_sn_bridge_gpio_free(struct gpio_chip *chip, unsigned int offset)
nit: did you need two blank lines before this function?
No.
@@ -1500,6 +1829,12 @@ static int ti_sn65dsi86_probe(struct i2c_client *client, return ret; }
if (IS_ENABLED(CONFIG_PWM)) {ret = ti_sn65dsi86_add_aux_device(pdata, &pdata->pwm_aux, "pwm");if (ret)return ret;}nit: also update the comment block above that says "Soon the PWM provided by the bridge chip..."
Missed that one, thanks.
Regards, Bjorn