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

[Paul Cercueil]

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).

-Paul