linux-kernel - Re: [PATCH 2/2] pwm: bcm-kona: apply pwm settings on enable

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Date:   Thu, 8 Nov 2018 07:32:02 -0800
From:   Tim Kryger <tim.kryger@...il.com>
To:     u.kleine-koenig@...gutronix.de
Cc:     peron.clem@...il.com, Thierry Reding <thierry.reding@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Linux PWM <linux-pwm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        suji.velupillai@...adcom.com, Tim Kryger <tim.kryger@...aro.org>
Subject: Re: [PATCH 2/2] pwm: bcm-kona: apply pwm settings on enable

On Thu, Nov 8, 2018 at 2:59 AM Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
>
> Hello,
>
> adding Tim Kryger as the initial author of the bcm-kona driver to Cc:.
> Maybe he can shed some light to the questions below?
>
> On Thu, Nov 08, 2018 at 11:47:17AM +0100, Clément Péron wrote:
> > On Wed, 7 Nov 2018 at 17:29, Uwe Kleine-König
> > <u.kleine-koenig@...gutronix.de> wrote:
> > > On Wed, Nov 07, 2018 at 10:36:13AM +0100, Clément Péron wrote:
> > > > From: Suji Velupillai <suji.velupillai@...adcom.com>
> > > >
> > > > When pwm_bl framework calls enable, a call to pwm_is_enabled(pwm) still
> > > > return false, this prevents the backlight being turn on at boot time.
> > > >
> > > > Signed-off-by: Suji Velupillai <suji.velupillai@...adcom.com>
> > > > Signed-off-by: Clément Péron <peron.clem@...il.com>
> > > > ---
> > > >  drivers/pwm/pwm-bcm-kona.c | 16 +++++++++++-----
> > > >  1 file changed, 11 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/drivers/pwm/pwm-bcm-kona.c b/drivers/pwm/pwm-bcm-kona.c
> > > > index 09a95aeb3a70..d991d53c4b38 100644
> > > > --- a/drivers/pwm/pwm-bcm-kona.c
> > > > +++ b/drivers/pwm/pwm-bcm-kona.c
> > > > @@ -108,8 +108,8 @@ static void kona_pwmc_apply_settings(struct kona_pwmc *kp, unsigned int chan)
> > > >       ndelay(400);
> > > >  }
> > > >
> > > > -static int kona_pwmc_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > > > -                         int duty_ns, int period_ns)
> > > > +static int __pwmc_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > > > +                      int duty_ns, int period_ns, bool pwmc_enabled)
> > > >  {
> > > >       struct kona_pwmc *kp = to_kona_pwmc(chip);
> > > >       u64 val, div, rate;
> > > > @@ -155,7 +155,7 @@ static int kona_pwmc_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > > >        * always calculated above to ensure the new values are
> > > >        * validated immediately instead of on enable.
> > > >        */
> > > > -     if (pwm_is_enabled(pwm)) {
> > > > +     if (pwm_is_enabled(pwm) || pwmc_enabled) {
> > >
> > > Having pwm-API-calls in hw-drivers is ugly. Apart from not giving the
> > > intended return code this function should IMHO be reserved to pwm
> > > consumers. The underlaying problem is that pwm-bl does:
> > >
> > >         pwm_config(pwm, duty_cycle, period);
> > >         pwm_enable(pwm);
> > >
> > > and expects that the duty_cycle and period is used then. Doesn't
> > > everything works just fine if the if-block is always executed?
> >
> > Tested and works fine for me. But I only have a Cygnus proc.
> > Maybe there is some issue with Kona as explained by the comment (even
> > if I don't understand it well).
> >
> >  * Don't apply settings if disabled. The period and duty cycle are
> >  * always calculated above to ensure the new values are
> >  * validated immediately instead of on enable.
>
> I wouldn't understand that as "If you apply settings on a disabled PWM a
> kitten dies". I think it only means: At the current point in time
> duty_cycle and period are not important as the hardware is off. So don't
> bother to write these values to the hardware.
>
> @Tim: Do you think (or can test if) there is a problem when doing
>
> -       if (pwm_is_enabled(pwm)) {
> +       if (1) {
>
> in kona_pwmc_config? (For sure the comment needs adaption and the if (1)
> shouldn't make it into the driver, just used that as shorthand for the
> change I want to suggest.)
>
> But still better than dropping the check is to convert the driver to the
> atomic API. With that this problem would simply not occur.
>
> Best regards
> Uwe
>
> --
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |

There is no per channel disable in the hardware so to simulate a
disable, duty is set to zero.

The check is there to prevent new config values from applying until
the channel is enabled.