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Date: Mon, 28 Nov 2022 15:39:11 +0100
From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To: Paul Cercueil <paul@...pouillou.net>
Cc: Thierry Reding <thierry.reding@...il.com>, 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
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 });
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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