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

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Message-ID: <CAHh=Yk85NHbm9eUKLm75GUP4gSP5eYFjVabTUXseyB6wHD4D=Q@mail.gmail.com>
Date:   Fri, 3 Feb 2023 16:06:38 +0800
From:   Nylon Chen <nylon.chen@...ive.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
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

Hi Uwe,

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.
are you mean state->period is a redundancy variable so we can use
ddata->real_period directly?

it seems reasonable, but I don't get your point, why do we need to
change the algorithm to DIV_ROUND_DOWN_ULL() and change the if-else
condition.

frac = DIV_ROUND_DOWN_ULL(num, ddata->real_period);
if (state->period < ddata->approx_period) {
    ...
}

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