Date:   Wed, 6 Nov 2019 22:24:39 +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>,
        Jernej Skrabec <jernej.skrabec@...l.net>
Subject: Re: [PATCH v3 4/7] pwm: sun4i: Add support to output source clock directly

Hi Uwe,

On Tue, 5 Nov 2019 at 15:57, Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
>
> On Tue, Nov 05, 2019 at 02:14:53PM +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 | 38 +++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 37 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> > index 54e19fa56a4e..810abf47c261 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 = state->period / 2;
>
> Please round up here.
Ok
>
> > +             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;
> > @@ -203,7 +222,8 @@ 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;
> > +     u32 ctrl, clk_rate;
> > +     bool bypass;
> >       int ret;
> >       unsigned int delay_us;
> >       unsigned long now;
> > @@ -218,6 +238,15 @@ 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".
> > +      */
> > +     clk_rate = clk_get_rate(sun4i_pwm->clk);
>
> clk_get_rate must not be called if the clk might be off.
Ok,

>
> > +     bypass = state->enabled &&
> > +              (state->period * clk_rate >= NSEC_PER_SEC) &&
>
> This is too coarse. With state->period = 1000000 this is fulfilled
> (unless the multiplication overflows).

Sorry, misunderstood the previous mail

What about something like this ?
((state->period - 1) * clk_rate <= NSEC_PER_SEC) &&
((state->period + 1) * clk_rate >= NSEC_PER_SEC) &&
 ((state->duty_cycle - 1) * 2 <= state->period) &&
 ((state->duty_cycle + 1) * 2 >= state->period);

We are sure that the user is looking for a PWM around the OSC with a
50% duty cycle ?

Regards,
Clement

>
> > +              (state->duty_cycle * 2 == state->period);
>
> This is too strict. See my previous mail about how this should be done.
>
> If bypass is true (and the hardware support it) you can skip the
> calculation of the other parameters.
Yes correct and also we can skip


>
> > +
> >       spin_lock(&sun4i_pwm->ctrl_lock);
> >       ctrl = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
> >
> > @@ -265,6 +294,13 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> >               ctrl &= ~BIT_CH(PWM_CLK_GATING, pwm->hwpwm);
> >       }
> >
> > +     if (sun4i_pwm->data->has_direct_mod_clk_output) {
> > +             if (bypass)
> > +                     ctrl |= BIT_CH(PWM_BYPASS, pwm->hwpwm);
> > +             else
> > +                     ctrl &= ~BIT_CH(PWM_BYPASS, pwm->hwpwm);
> > +     }
> > +
> >       sun4i_pwm_writel(sun4i_pwm, ctrl, PWM_CTRL_REG);
> >
> >       spin_unlock(&sun4i_pwm->ctrl_lock);
>
> Best regards
> Uwe
>
> --
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |

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