linux-kernel - Re: [PATCH v4 4/7] pwm: sun4i: Add support to output source clock directly

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Date:   Thu, 14 Nov 2019 23:47:00 +0100
From:   Clément Péron <peron.clem@...il.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Philipp Zabel <pza@...gutronix.de>, linux-pwm@...r.kernel.org,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        Jernej Skrabec <jernej.skrabec@...l.net>
Subject: Re: [PATCH v4 4/7] pwm: sun4i: Add support to output source clock directly

Hi Uwe,

On Wed, 13 Nov 2019 at 09:58, Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
>
> On Fri, Nov 08, 2019 at 09:45:14AM +0100, Clément Péron wrote:
> > From: Jernej Skrabec <jernej.skrabec@...l.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@...l.net>
> > Signed-off-by: Clément Péron <peron.clem@...il.com>
> > ---
> >  drivers/pwm/pwm-sun4i.c | 44 +++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 44 insertions(+)
> >
> > diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> > index a10022d6c0fd..9cc928ab47bc 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@...e-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);
>
> I first thought you're losing precision here by reusing state->period
> here, but with a divisor of 2 everything is fine.
>
> > +             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;
> > @@ -204,6 +223,7 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> >       struct sun4i_pwm_chip *sun4i_pwm = to_sun4i_pwm_chip(chip);
> >       struct pwm_state cstate;
> >       u32 ctrl;
> > +     bool bypass = false;
> >       int ret;
> >       unsigned int delay_us;
> >       unsigned long now;
> > @@ -218,9 +238,24 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> >               }
> >       }
> >
> > +     /*
> > +      * Although it would make much more sense to check for bypass in
> > +      * sun4i_pwm_calculate(), value of bypass bit also depends on "enabled".
>
> I don't understand this reasoning. sun4i_pwm_calculate knows about
> .enabled and also sun4i_pwm->data->has_direct_mod_clk_output. Maybe just
> add a bool *bypass as parameter and move the logic there?

I asked myself the same question, however the sun4i_pwm_calculate is
only called when period or duty_cycle is updated not when state is
toggled between disabled / enabled.

Should we also call it when the state is switched to enabled ?

Regards,
Clément

>
> > +      */
> > +     if (state->enabled) {
> > +             u32 clk_rate = clk_get_rate(sun4i_pwm->clk);
> > +             bypass = (state->period * clk_rate >= NSEC_PER_SEC) &&
> > +                      (state->period * clk_rate < 2 * NSEC_PER_SEC) &&
> > +                      (state->duty_cycle * clk_rate * 2 >= NSEC_PER_SEC);
> > +     }
> > +
>
> This looks right now.
>
> Best regards
> Uwe
>
> --
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | https://www.pengutronix.de/ |