linux-kernel - Re: [PATCH v3 2/2] pwm: sifive: change the PWM controlled LED algorithm

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Message-ID: <CAJM55Z9TPVyJyWwWAS2FznSc8FvnR7qMxQ412eMzBkD=5abp6g@mail.gmail.com>
Date:   Thu, 20 Apr 2023 12:46:10 +0200
From:   Emil Renner Berthing <emil.renner.berthing@...onical.com>
To:     Nylon Chen <nylon.chen@...ive.com>
Cc:     aou@...s.berkeley.edu, conor@...nel.org, 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, u.kleine-koenig@...gutronix.de,
        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, Conor Dooley <conor.dooley@...rochip.com>
Subject: Re: [PATCH v3 2/2] pwm: sifive: change the PWM controlled LED algorithm

On Thu, 20 Apr 2023 at 12:41, Nylon Chen <nylon.chen@...ive.com> wrote:
>
> Hi, Emil
>
> Emil Renner Berthing <emil.renner.berthing@...onical.com> 於 2023年4月20日 週四 下午6:04寫道：
> >
> > On Thu, 20 Apr 2023 at 11:35, Nylon Chen <nylon.chen@...ive.com> 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]
> > >
> > > Link: https://sifive.cdn.prismic.io/sifive/1a82e600-1f93-4f41-b2d8-86ed8b16acba_fu740-c000-manual-v1p6.pdf [0]
> > >
> > > Acked-by: Conor Dooley <conor.dooley@...rochip.com>
> > > Reviewed-by: Conor Dooley <conor.dooley@...rochip.com>
> > > Signed-off-by: Nylon Chen <nylon.chen@...ive.com>
> > > Signed-off-by: Vincent Chen <vincent.chen@...ive.com>
> > > ---
> > >  drivers/pwm/pwm-sifive.c | 9 ++++++---
> > >  1 file changed, 6 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c
> > > index 393a4b97fc19..d5d5f36da297 100644
> > > --- a/drivers/pwm/pwm-sifive.c
> > > +++ b/drivers/pwm/pwm-sifive.c
> > > @@ -132,13 +132,13 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > >  {
> > >         struct pwm_sifive_ddata *ddata = pwm_sifive_chip_to_ddata(chip);
> > >         struct pwm_state cur_state;
> > > -       unsigned int duty_cycle;
> > > +       unsigned int duty_cycle, period;
> > >         unsigned long long num;
> > >         bool enabled;
> > >         int ret = 0;
> > >         u32 frac;
> > >
> > > -       if (state->polarity != PWM_POLARITY_INVERSED)
> > > +       if (state->polarity != PWM_POLARITY_NORMAL && state->polarity != PWM_POLARITY_INVERSED)
> > >                 return -EINVAL;
> > >
> > >         cur_state = pwm->state;
> > > @@ -154,10 +154,13 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > >          * calculating the register values first and then writing them
> > >          * consecutively
> > >          */
> > > +       period = max(state->period, ddata->approx_period);
> >
> > Hi Nylon,
> >
> > I don't understand this patch. You introduce this new variable,
> > period, and set it here but you never seem to use it. If you planned
> > to use it instead of state->period below, why should it be the max of
> > the old period and what is requested? What happens if the consumer
> > wants to lower the period?
> Sorry this was an oversight on my part, there was a line correction that didn't change to
> - frac = DIV64_U64_ROUND_CLOSEST(num, state->period);
> + frac = DIV64_U64_ROUND_CLOSEST(num, period);

I see, so then my second question was why period needs to be the
larger of the previous period and the requested period.

What happens if the requested period, state->period, is lower than the
old period, ddata->approx_period? Then the period will be changed to
state->period below, but the calculations will be made using period =
ddata->approx_period, right?

> >
> > Also above you now allow both PWM_POLARITY_NORMAL and
> > PWM_POLARITY_INVERSED but you treat both cases the same.
> I may have misunderstood what Uwe means here, I will confirm again here
> >
> > /Emil
> >
> > >         num = (u64)duty_cycle * (1U << PWM_SIFIVE_CMPWIDTH);
> > >         frac = DIV64_U64_ROUND_CLOSEST(num, state->period);
> > > -       /* The hardware cannot generate a 100% duty cycle */
> > >         frac = min(frac, (1U << PWM_SIFIVE_CMPWIDTH) - 1);
> > > +       /* The hardware cannot generate a 100% duty cycle */
> > > +       frac = (1U << PWM_SIFIVE_CMPWIDTH) - 1 - frac;
> > > +
> > >
> > >         mutex_lock(&ddata->lock);
> > >         if (state->period != ddata->approx_period) {
> > > --
> > > 2.40.0
> > >