Re: [PATCH 2/5] pwm: jz4740: Fix pin level of disabled TCU2 channels, part 2

[Uwe Kleine-König]

Hello,

On Tue, Oct 25, 2022 at 11:10:46AM +0100, Paul Cercueil wrote:
> Le mar. 25 oct. 2022 à 08:44:10 +0200, Uwe Kleine-König
> <u.kleine-koenig@xxxxxxxxxxxxxx> a écrit :
> > On Mon, Oct 24, 2022 at 09:52:10PM +0100, Paul Cercueil wrote:
> > >  After commit a020f22a4ff5 ("pwm: jz4740: Make PWM start with the
> > > active part"),
> > >  the trick to set duty > period to properly shut down TCU2 channels
> > > did
> > >  not work anymore, because of the polarity inversion.
> > > 
> > >  Address this issue by restoring the proper polarity before
> > > disabling the
> > >  channels.
> > > 
> > >  Fixes: a020f22a4ff5 ("pwm: jz4740: Make PWM start with the active
> > > part")
> > >  Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
> > >  Cc: stable@xxxxxxxxxxxxxxx
> > >  ---
> > >   drivers/pwm/pwm-jz4740.c | 62
> > > ++++++++++++++++++++++++++--------------
> > >   1 file changed, 40 insertions(+), 22 deletions(-)
> > > 
> > >  diff --git a/drivers/pwm/pwm-jz4740.c b/drivers/pwm/pwm-jz4740.c
> > >  index 228eb104bf1e..65462a0052af 100644
> > >  --- a/drivers/pwm/pwm-jz4740.c
> > >  +++ b/drivers/pwm/pwm-jz4740.c
> > >  @@ -97,6 +97,19 @@ static int jz4740_pwm_enable(struct pwm_chip
> > > *chip, struct pwm_device *pwm)
> > >   	return 0;
> > >   }
> > > 
> > >  +static void jz4740_pwm_set_polarity(struct jz4740_pwm_chip *jz,
> > >  +				    unsigned int hwpwm,
> > >  +				    enum pwm_polarity polarity)
> > >  +{
> > >  +	unsigned int value = 0;
> > >  +
> > >  +	if (polarity == PWM_POLARITY_INVERSED)
> > >  +		value = TCU_TCSR_PWM_INITL_HIGH;
> > >  +
> > >  +	regmap_update_bits(jz->map, TCU_REG_TCSRc(hwpwm),
> > >  +			   TCU_TCSR_PWM_INITL_HIGH, value);
> > >  +}
> > >  +
> > >   static void jz4740_pwm_disable(struct pwm_chip *chip, struct
> > > pwm_device *pwm)
> > >   {
> > >   	struct jz4740_pwm_chip *jz = to_jz4740(chip);
> > >  @@ -130,6 +143,7 @@ static int jz4740_pwm_apply(struct pwm_chip
> > > *chip, struct pwm_device *pwm,
> > >   	unsigned long long tmp = 0xffffull * NSEC_PER_SEC;
> > >   	struct clk *clk = pwm_get_chip_data(pwm);
> > >   	unsigned long period, duty;
> > >  +	enum pwm_polarity polarity;
> > >   	long rate;
> > >   	int err;
> > > 
> > >  @@ -169,6 +183,9 @@ static int jz4740_pwm_apply(struct pwm_chip
> > > *chip, struct pwm_device *pwm,
> > >   	if (duty >= period)
> > >   		duty = period - 1;
> > > 
> > >  +	/* Restore regular polarity before disabling the channel. */
> > >  +	jz4740_pwm_set_polarity(jz4740, pwm->hwpwm, state->polarity);
> > >  +
> > 
> > Does this introduce a glitch?
> 
> Maybe. But the PWM is shut down before finishing its period anyway, so there
> was already a glitch.
> 
> > >   	jz4740_pwm_disable(chip, pwm);
> > > 
> > >   	err = clk_set_rate(clk, rate);
> > >  @@ -190,29 +207,30 @@ static int jz4740_pwm_apply(struct pwm_chip
> > > *chip, struct pwm_device *pwm,
> > >   	regmap_update_bits(jz4740->map, TCU_REG_TCSRc(pwm->hwpwm),
> > >   			   TCU_TCSR_PWM_SD, TCU_TCSR_PWM_SD);
> > > 
> > >  -	/*
> > >  -	 * Set polarity.
> > >  -	 *
> > >  -	 * The PWM starts in inactive state until the internal timer
> > > reaches the
> > >  -	 * duty value, then becomes active until the timer reaches the
> > > period
> > >  -	 * value. In theory, we should then use (period - duty) as the
> > > real duty
> > >  -	 * value, as a high duty value would otherwise result in the PWM
> > > pin
> > >  -	 * being inactive most of the time.
> > >  -	 *
> > >  -	 * Here, we don't do that, and instead invert the polarity of the
> > > PWM
> > >  -	 * when it is active. This trick makes the PWM start with its
> > > active
> > >  -	 * state instead of its inactive state.
> > >  -	 */
> > >  -	if ((state->polarity == PWM_POLARITY_NORMAL) ^ state->enabled)
> > >  -		regmap_update_bits(jz4740->map, TCU_REG_TCSRc(pwm->hwpwm),
> > >  -				   TCU_TCSR_PWM_INITL_HIGH, 0);
> > >  -	else
> > >  -		regmap_update_bits(jz4740->map, TCU_REG_TCSRc(pwm->hwpwm),
> > >  -				   TCU_TCSR_PWM_INITL_HIGH,
> > >  -				   TCU_TCSR_PWM_INITL_HIGH);
> > >  -
> > >  -	if (state->enabled)
> > >  +	if (state->enabled) {
> > >  +		/*
> > >  +		 * Set polarity.
> > >  +		 *
> > >  +		 * The PWM starts in inactive state until the internal timer
> > >  +		 * reaches the duty value, then becomes active until the timer
> > >  +		 * reaches the period value. In theory, we should then use
> > >  +		 * (period - duty) as the real duty value, as a high duty value
> > >  +		 * would otherwise result in the PWM pin being inactive most of
> > >  +		 * the time.
> > >  +		 *
> > >  +		 * Here, we don't do that, and instead invert the polarity of
> > >  +		 * the PWM when it is active. This trick makes the PWM start
> > >  +		 * with its active state instead of its inactive state.
> > >  +		 */
> > >  +		if (state->polarity == PWM_POLARITY_NORMAL)
> > >  +			polarity = PWM_POLARITY_INVERSED;
> > >  +		else
> > >  +			polarity = PWM_POLARITY_NORMAL;
> > >  +
> > >  +		jz4740_pwm_set_polarity(jz4740, pwm->hwpwm, polarity);
> > >  +
> > >   		jz4740_pwm_enable(chip, pwm);
> > >  +	}
> > 
> > Note that for disabled PWMs there is no official guaranty about the pin
> > state. So it would be ok (but admittedly not great) to simplify the
> > driver and accept that the pinstate is active while the PWM is off.
> > IMHO this is also better than a glitch.
> > 
> > If a consumer wants the PWM to be in its inactive state, they should
> > not disable it.
> 
> Completely disagree. I absolutely do not want the backlight to go full
> bright mode when the PWM pin is disabled. And disabling the backlight is a
> thing (for screen blanking and during mode changes).

For some hardwares there is no pretty choice. So the gist is: If the
backlight driver wants to ensure that the PWM pin is driven to its
inactive level, it should use:

	pwm_apply(pwm, { .period = ..., .duty_cycle = 0, .enabled = true });

and better not

	pwm_apply(pwm, { ..., .enabled = false });

Best regards
Uwe

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

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