linux-kernel - Re: [PATCH v2 2/2] pwm: sifive: change the PWM controlled LED algorithm

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Date:   Wed, 1 Mar 2023 10:20:50 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Nylon Chen <nylon.chen@...ive.com>
Cc:     aou@...s.berkeley.edu, conor@...nel.org,
        emil.renner.berthing@...onical.com, geert+renesas@...der.be,
        heiko@...ech.de, krzysztof.kozlowski+dt@...aro.org,
        palmer@...belt.com, paul.walmsley@...ive.com, robh+dt@...nel.org,
        thierry.reding@...il.com, devicetree@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, nylon7717@...il.com,
        zong.li@...ive.com, greentime.hu@...ive.com,
        vincent.chen@...ive.com
Subject: Re: [PATCH v2 2/2] pwm: sifive: change the PWM controlled LED
 algorithm

Hello Nylon,

On Wed, Feb 01, 2023 at 04:56:42PM +0800, Nylon Chen wrote:
> Uwe Kleine-König <u.kleine-koenig@...gutronix.de> 於 2023年1月30日 週一 下午6:17寫道：
> > On Mon, Jan 30, 2023 at 05:32:29PM +0800, Nylon Chen wrote:
> > > The `frac` variable represents the pulse inactive time, and the result of
> > > this algorithm is the pulse active time. Therefore, we must reverse the
> > > result.
> > >
> > > The reference is SiFive FU740-C000 Manual[0].
> > >
> > > [0]: https://sifive.cdn.prismic.io/sifive/1a82e600-1f93-4f41-b2d8-86ed8b16acba_fu740-c000-manual-v1p6.pdf
> > >
> > > Signed-off-by: Nylon Chen <nylon.chen@...ive.com>
> > > ---
> > >  drivers/pwm/pwm-sifive.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c
> > > index 62b6acc6373d..a5eda165d071 100644
> > > --- a/drivers/pwm/pwm-sifive.c
> > > +++ b/drivers/pwm/pwm-sifive.c
> > > @@ -158,6 +158,7 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > >       frac = DIV64_U64_ROUND_CLOSEST(num, state->period);
> > >       /* The hardware cannot generate a 100% duty cycle */
> > >       frac = min(frac, (1U << PWM_SIFIVE_CMPWIDTH) - 1);
> > > +     frac = (1U << PWM_SIFIVE_CMPWIDTH) - 1 - frac;
> >
> > The same problem exists in pwm_sifive_get_state(), doesn't it?
> >
> > As fixing this is an interruptive change anyhow, this is the opportunity
> > to align the driver to the rules tested by PWM_DEBUG.
> >
> > The problems I see in the driver (only checked quickly, so I might be
> > wrong):
> >
> >  - state->period != ddata->approx_period isn't necessarily a problem. If
> >    state->period > ddata->real_period that's fine and the driver should
> >    continue
> >
> >  - frac = DIV64_U64_ROUND_CLOSEST(num, state->period);
> >    is wrong for two reasons:
> >    it should round down and use the real period.
> >
> I need a little time to clarify your assumptions. If possible, I will
> make similar changes.
> 
> e.g.
> rounddown(num, state->period);
> if (state->period < ddata->approx_period)
>     ...

the idea is that for a given request apply should do the following to
select the hardware setting:

 - Check polarity, if the hardware doesn't support it, return -EINVAL.
   (A period always starts with the active phase for the duration of
   duty_cycle. For normal polarity active = high.)
 - Pick the biggest period length possible that is not bigger than the
   requested period.
 - For the picked period, select the biggest duty_cycle possible that is
   not bigger than the requested duty_cycle.

Then if possible switch to the selected setting in an atomic step.

Does this clearify your doubts?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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