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Date: Tue, 29 Nov 2022 12:34:44 +0000
From: Paul Cercueil <paul@...pouillou.net>
To: Thierry Reding <thierry.reding@...il.com>
Cc: Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>, od@...ndingux.net,
linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-mips@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 2/5] pwm: jz4740: Fix pin level of disabled TCU2 channels,
part 2
Hi Thierry,
Le mar. 29 nov. 2022 à 13:16:05 +0100, Thierry Reding
<thierry.reding@...il.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@...gutronix.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@...pouillou.net>
>> > > > Cc: stable@...r.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
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