Re: [PATCH v3 2/2] drm/bridge: ti-sn65dsi86: Implement the pwm_chip

On Mon, Jun 21, 2021 at 10:09:48PM -0500, Bjorn Andersson wrote:

The SN65DSI86 provides the ability to supply a PWM signal on GPIO 4, with the primary purpose of controlling the backlight of the attached panel. Add an implementation that exposes this using the standard PWM framework, to allow e.g. pwm-backlight to expose this to the user.

Signed-off-by: Bjorn Andersson bjorn.andersson@linaro.org

Changes since v2:

• Corrected calculation of scale, to include a 1 instead of 1/NSEC_TO_SEC and rounded the period up in get_state, to make sure its idempotent
• Changed duty_cycle calculation to make sure it idempotent over my tested period
• Documented "Limitations"
• Documented muxing operation after pm_runtime_get_sync()

drivers/gpu/drm/bridge/ti-sn65dsi86.c | 335 +++++++++++++++++++++++++- 1 file changed, 334 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c index 5d712c8c3c3b..0eabbdad1830 100644 --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c @@ -4,6 +4,7 @@

• datasheet: https://www.ti.com/lit/ds/symlink/sn65dsi86.pdf

*/

+#include <linux/atomic.h> #include <linux/auxiliary_bus.h> #include <linux/bits.h> #include <linux/clk.h> @@ -15,6 +16,7 @@ #include <linux/module.h> #include <linux/of_graph.h> #include <linux/pm_runtime.h> +#include <linux/pwm.h> #include <linux/regmap.h> #include <linux/regulator/consumer.h>

@@ -91,6 +93,13 @@ #define SN_ML_TX_MODE_REG 0x96 #define ML_TX_MAIN_LINK_OFF 0 #define ML_TX_NORMAL_MODE BIT(0) +#define SN_PWM_PRE_DIV_REG 0xA0 +#define SN_BACKLIGHT_SCALE_REG 0xA1 +#define BACKLIGHT_SCALE_MAX 0xFFFF +#define SN_BACKLIGHT_REG 0xA3 +#define SN_PWM_EN_INV_REG 0xA5 +#define SN_PWM_INV_MASK BIT(0) +#define SN_PWM_EN_MASK BIT(1) #define SN_AUX_CMD_STATUS_REG 0xF4 #define AUX_IRQ_STATUS_AUX_RPLY_TOUT BIT(3) #define AUX_IRQ_STATUS_AUX_SHORT BIT(5) @@ -113,11 +122,14 @@

#define SN_LINK_TRAINING_TRIES 10

+#define SN_PWM_GPIO_IDX 3 /* 4th GPIO */

/**

• struct ti_sn65dsi86 - Platform data for ti-sn65dsi86 driver.
• @bridge_aux: AUX-bus sub device for MIPI-to-eDP bridge functionality.
• @gpio_aux: AUX-bus sub device for GPIO controller functionality.
• @aux_aux: AUX-bus sub device for eDP AUX channel functionality.

• • @pwm_aux: AUX-bus sub device for PWM controller functionality.
  • @dev: Pointer to the top level (i2c) device.
  • @regmap: Regmap for accessing i2c.

@@ -145,11 +157,17 @@

•            bitmap so we can do atomic ops on it without an extra
  

•            lock so concurrent users of our 4 GPIOs don't stomp on
  

•            each other's read-modify-write.
  

• • @pchip: pwm_chip if the PWM is exposed.
• • @pwm_enabled: Used to track if the PWM signal is currently enabled.
• • @pwm_refclk_freq: Cache for the reference clock input to the PWM.
• • @pwm_pin_busy: Track if GPIO4 is currently requested for GPIO or PWM.
  */

struct ti_sn65dsi86 { struct auxiliary_device bridge_aux; struct auxiliary_device gpio_aux; struct auxiliary_device aux_aux;

• struct auxiliary_device pwm_aux;

  struct device *dev; struct regmap *regmap;

@@ -172,6 +190,12 @@ struct ti_sn65dsi86 { struct gpio_chip gchip; DECLARE_BITMAP(gchip_output, SN_NUM_GPIOS); #endif +#if defined(CONFIG_PWM)

• struct pwm_chip pchip;
• bool pwm_enabled;
• unsigned int pwm_refclk_freq;
• atomic_t pwm_pin_busy;

+#endif };

static const struct regmap_range ti_sn65dsi86_volatile_ranges[] = { @@ -190,6 +214,25 @@ static const struct regmap_config ti_sn65dsi86_regmap_config = { .cache_type = REGCACHE_NONE, };

+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) { @@ -253,6 +296,14 @@ 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 }

static void ti_sn65dsi86_enable_comms(struct ti_sn65dsi86 *pdata) @@ -1044,6 +1095,258 @@ static int ti_sn_bridge_parse_dsi_host(struct ti_sn65dsi86 *pdata) return 0; }

+#if defined(CONFIG_PWM) +static int ti_sn_pwm_pin_request(struct ti_sn65dsi86 *pdata) +{

• return atomic_xchg(&pdata->pwm_pin_busy, 1) ? -EBUSY : 0;

+}

+static void ti_sn_pwm_pin_release(struct ti_sn65dsi86 *pdata) +{

• atomic_set(&pdata->pwm_pin_busy, 0);

+}

+static struct ti_sn65dsi86 *pwm_chip_to_ti_sn_bridge(struct pwm_chip *chip) +{

• return container_of(chip, struct ti_sn65dsi86, pchip);

+}

+static int ti_sn_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm) +{

• struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
• return ti_sn_pwm_pin_request(pdata);

+}

+static void ti_sn_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm) +{

• struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
• ti_sn_pwm_pin_release(pdata);

+}

+/*

• • 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.
• • • Changing both period and duty_cycle is not done atomically, so the output
• • might briefly be a mix of the two settings.
• */

+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 tick;
• int ret;
• if (!pdata->pwm_enabled) {

• ret = pm_runtime_get_sync(pdata->dev);
  

• if (ret < 0)
  

• 	return ret;
  

• /*
  

•  * 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;
  

• }
  

In reply to your v2 I requested to short-cut the case !pdata->pwm_enabled && !state->enabled without enabling stuff.

• }
• if (state->enabled) {

• /*
  

•  * 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 (1 * 1 + 1) / REFCLK_FREQ */
  

• if (state->period * pdata->pwm_refclk_freq <= 2 * NSEC_PER_SEC) {
  

• 	ret = -EINVAL;
  

• 	goto out;
  

• }
  

• pre_div = DIV_ROUND_UP((state->period * pdata->pwm_refclk_freq - NSEC_PER_SEC),
  

• 		       (NSEC_PER_SEC * BACKLIGHT_SCALE_MAX));
  

• if (pre_div > 0xff)
  

• 	pre_div = 0xff;
  

• scale = (state->period * pdata->pwm_refclk_freq - NSEC_PER_SEC) / (NSEC_PER_SEC * pre_div);
  

Please consider this multiplication to overflow. Something like:

if (state->period > $someterm) period = $someterm; else period = state->period;

is usually appropriate. Also NSEC_PER_SEC * pre_div might overflow. Moreover to divide a u64 you must not rely on / but need do_div() or some variant of it.

• /*
  

•  * PWM duty cycle is given as:
  

•  *
  

•  *   duty = BACKLIGHT / (BACKLIGHT_SCALE + 1)
  

•  *
  

•  * The documentation is however inconsistent in its examples,
  

•  * so the interpretation used here is that the duty cycle is
  

•  * the period of BACKLIGHT * PRE_DIV / REFCLK_FREQ.
  

I don't understand this.

•  *
  

•  * The ratio PRE_DIV / REFCLK_FREQ is rounded up to whole
  

•  * nanoseconds in order to ensure that the calculations are
  

•  * idempotent and gives results that are smaller than the
  

•  * requested value.
  

•  */
  

• tick = DIV_ROUND_UP(NSEC_PER_SEC * pre_div, pdata->pwm_refclk_freq);
  

• backlight = state->duty_cycle / tick;
  

You're loosing precision here by dividing by the result of a division.

• 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) |

•      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;

+out:

• if (!pdata->pwm_enabled)

• pm_runtime_put_sync(pdata->dev);
  

• return ret;

+}

+static void ti_sn_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,

• 		struct pwm_state *state)
  

+{

• struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
• unsigned int pwm_en_inv;
• unsigned int pre_div;
• u16 backlight;
• u16 scale;
• int ret;
• ret = regmap_read(pdata->regmap, SN_PWM_EN_INV_REG, &pwm_en_inv);
• if (ret)

• return;
  

• ret = ti_sn65dsi86_read_u16(pdata, SN_BACKLIGHT_SCALE_REG, &scale);
• if (ret)

• return;
  

• ret = ti_sn65dsi86_read_u16(pdata, SN_BACKLIGHT_REG, &backlight);
• if (ret)

• return;
  

• ret = regmap_read(pdata->regmap, SN_PWM_PRE_DIV_REG, &pre_div);
• if (ret)

• return;
  

• state->enabled = FIELD_GET(SN_PWM_EN_MASK, pwm_en_inv);
• if (FIELD_GET(SN_PWM_INV_MASK, pwm_en_inv))

• state->polarity = PWM_POLARITY_INVERSED;
  

• else

• state->polarity = PWM_POLARITY_NORMAL;
  

• state->period = DIV_ROUND_UP(NSEC_PER_SEC * (pre_div * scale + 1), pdata->pwm_refclk_freq);
• state->duty_cycle = backlight * DIV_ROUND_UP(NSEC_PER_SEC * pre_div, pdata->pwm_refclk_freq);

If you use

state->duty_cycle = DIV_ROUND_UP(backlight * NSEC_PER_SEC * pre_div, pdata->pwm_refclk_freq);

instead (with a cast to u64 to not yield an overflow) the result is more exact.

I still find this surprising, I'd expect that SCALE also matters for the duty_cycle. With the assumption implemented here modifying SCALE only affects the period. This should be easy to verify?! I would expect that changing SCALE doesn't affect the relative duty_cycle, so the brightness of an LED is unaffected (unless the period gets too big of course).

I didn't spend much cycles to verify that the logic in .apply() matches .get_state(). I'd keep that check for the next iteration.

Best regards Uwe