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

[Paul Cercueil <paul@crapouillou.net>]

Hi Thierry,

Le mar. 29 nov. 2022 à 13:16:05 +0100, Thierry Reding 
<thierry.reding@gmail.com> a écrit :
> On Mon, Nov 28, 2022 at 03:39:11PM +0100, Uwe Kleine-König wrote:
>>  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@pengutronix.de> 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@crapouillou.net>
>>  > > >  Cc: stable@vger.kernel.org
>>  > > >  ---
>>  > > >   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 });
> 
> Depending on your hardware capabilities you may also be able to use
> pinctrl to configure the pin to behave properly when the PWM is
> disabled. Not all hardware can do that, though.

Been there, done that. It got refused.
https://lkml.org/lkml/2019/5/22/607

Cheers,
-Paul