Re: [PATCH v8 5/6] pwm: sun4i: Add support to output source clock directly

["Clément Péron" <peron.clem@gmail.com>]

Hi Uwe,

On Thu, 21 Nov 2019 at 22:21, Clément Péron <peron.clem@gmail.com> wrote:
>
> Hi Uwe,
>
> On Thu, 21 Nov 2019 at 22:16, Uwe Kleine-König
> <u.kleine-koenig@pengutronix.de> wrote:
> >
> > On Thu, Nov 21, 2019 at 08:59:01PM +0100, Clément Péron wrote:
> > > From: Jernej Skrabec <jernej.skrabec@siol.net>
> > >
> > > PWM core has an option to bypass whole logic and output unchanged source
> > > clock as PWM output. This is achieved by enabling bypass bit.
> > >
> > > Note that when bypass is enabled, no other setting has any meaning, not
> > > even enable bit.
> > >
> > > This mode of operation is needed to achieve high enough frequency to
> > > serve as clock source for AC200 chip which is integrated into same
> > > package as H6 SoC.
> > >
> > > Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
> > > Signed-off-by: Clément Péron <peron.clem@gmail.com>
> > > ---
> > >  drivers/pwm/pwm-sun4i.c | 48 +++++++++++++++++++++++++++++++++++++++--
> > >  1 file changed, 46 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> > > index 1fa2057419fb..0fe9c680d6d0 100644
> > > --- a/drivers/pwm/pwm-sun4i.c
> > > +++ b/drivers/pwm/pwm-sun4i.c
> > > @@ -3,6 +3,10 @@
> > >   * Driver for Allwinner sun4i Pulse Width Modulation Controller
> > >   *
> > >   * Copyright (C) 2014 Alexandre Belloni <alexandre.belloni@free-electrons.com>
> > > + *
> > > + * Limitations:
> > > + * - When outputing the source clock directly, the PWM logic will be bypassed
> > > + *   and the currently running period is not guaranteed to be completed
> > >   */
> > >
> > >  #include <linux/bitops.h>
> > > @@ -73,6 +77,7 @@ static const u32 prescaler_table[] = {
> > >
> > >  struct sun4i_pwm_data {
> > >       bool has_prescaler_bypass;
> > > +     bool has_direct_mod_clk_output;
> > >       unsigned int npwm;
> > >  };
> > >
> > > @@ -118,6 +123,20 @@ static void sun4i_pwm_get_state(struct pwm_chip *chip,
> > >
> > >       val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
> > >
> > > +     /*
> > > +      * PWM chapter in H6 manual has a diagram which explains that if bypass
> > > +      * bit is set, no other setting has any meaning. Even more, experiment
> > > +      * proved that also enable bit is ignored in this case.
> > > +      */
> > > +     if ((val & BIT_CH(PWM_BYPASS, pwm->hwpwm)) &&
> > > +         sun4i_pwm->data->has_direct_mod_clk_output) {
> > > +             state->period = DIV_ROUND_UP_ULL(NSEC_PER_SEC, clk_rate);
> > > +             state->duty_cycle = DIV_ROUND_UP_ULL(state->period, 2);
> > > +             state->polarity = PWM_POLARITY_NORMAL;
> > > +             state->enabled = true;
> > > +             return;
> > > +     }
> > > +
> > >       if ((PWM_REG_PRESCAL(val, pwm->hwpwm) == PWM_PRESCAL_MASK) &&
> > >           sun4i_pwm->data->has_prescaler_bypass)
> > >               prescaler = 1;
> > > @@ -149,13 +168,24 @@ static void sun4i_pwm_get_state(struct pwm_chip *chip,
> > >
> > >  static int sun4i_pwm_calculate(struct sun4i_pwm_chip *sun4i_pwm,
> > >                              const struct pwm_state *state,
> > > -                            u32 *dty, u32 *prd, unsigned int *prsclr)
> > > +                            u32 *dty, u32 *prd, unsigned int *prsclr,
> > > +                            bool *bypass)
> > >  {
> > >       u64 clk_rate, div = 0;
> > >       unsigned int pval, prescaler = 0;
> > >
> > >       clk_rate = clk_get_rate(sun4i_pwm->clk);
> > >
> > > +     *bypass = sun4i_pwm->data->has_direct_mod_clk_output &&
> > > +               state->enabled &&
> > > +               (state->period * clk_rate >= NSEC_PER_SEC) &&
> > > +               (state->period * clk_rate < 2 * NSEC_PER_SEC) &&
> > > +               (state->duty_cycle * clk_rate * 2 >= NSEC_PER_SEC);
> > > +
> > > +     /* Skip calculation of other parameters if we bypass them */
> > > +     if (*bypass)
> > > +             return 0;
> > > +
> > >       if (sun4i_pwm->data->has_prescaler_bypass) {
> > >               /* First, test without any prescaler when available */
> > >               prescaler = PWM_PRESCAL_MASK;
> > > @@ -206,6 +236,7 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > >       int ret;
> > >       unsigned int delay_us, prescaler;
> > >       unsigned long now;
> > > +     bool bypass;
> > >
> > >       pwm_get_state(pwm, &cstate);
> > >
> > > @@ -220,7 +251,8 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > >       spin_lock(&sun4i_pwm->ctrl_lock);
> > >       ctrl = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
> > >
> > > -     ret = sun4i_pwm_calculate(sun4i_pwm, state, &duty, &period, &prescaler);
> > > +     ret = sun4i_pwm_calculate(sun4i_pwm, state, &duty, &period, &prescaler,
> > > +                               &bypass);
> > >       if (ret) {
> > >               dev_err(chip->dev, "period exceeds the maximum value\n");
> > >               spin_unlock(&sun4i_pwm->ctrl_lock);
> > > @@ -229,6 +261,18 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > >               return ret;
> > >       }
> > >
> > > +     if (sun4i_pwm->data->has_direct_mod_clk_output) {
> > > +             if (bypass) {
> > > +                     ctrl |= BIT_CH(PWM_BYPASS, pwm->hwpwm);
> > > +                     /* We can skip other parameter */
> > > +                     sun4i_pwm_writel(sun4i_pwm, ctrl, PWM_CTRL_REG);
> > > +                     spin_unlock(&sun4i_pwm->ctrl_lock);
> > > +                     return 0;
> > > +             } else {
> > > +                     ctrl &= ~BIT_CH(PWM_BYPASS, pwm->hwpwm);
> > > +             }
> > > +     }
> >
> > This could be simplified to:
> >
> >         if (bypass) {
> >                 ctrl |= BIT_CH(PWM_BYPASS, pwm->hwpwm);
> >                 /*
> >                  * Other parameters are not relevant in this mode and so
> >                  * writing them can be skipped
> >                  */
> >                 sun4i_pwm_writel(sun4i_pwm, ctrl, PWM_CTRL_REG);
> >                 spin_unlock(&sun4i_pwm->ctrl_lock);
> >                 return 0;
> >         } else {
> >                 ctrl &= ~BIT_CH(PWM_BYPASS, pwm->hwpwm);
> >         }
> >
> > which has the advantage(?) that the bypass bit is also (more obviously)
> > cleared for SoCs that don't support it and it reduces the indention
> > level.
>
> This bit is not guaranteed to be reserved for all the SoC variants.
>
> I don't think it's a good idea to set to 0 a bit which is undefined.

Let me know if you agree or not with this and I send the v9 according
to your answer.

Regards,
Clément

>
> Regards,
> Clement
>
> >
> > Best regards
> > Uwe
> >
> > --
> > Pengutronix e.K.                           | Uwe Kleine-König            |
> > Industrial Linux Solutions                 | https://www.pengutronix.de/ |