[PATCH v2 2/2] pwm: sifive: change the PWM controlled LED algorithm

[Nylon Chen]

Hi Uwe,

Sorry it's so long ago.

I have completed the implementation of the new version, but there is
one thing about this letter that I still don't quite understand.

Uwe Kleine-König <u.kleine-koenig at pengutronix.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 at sifive.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
>
I still don’t quite understand the description of this paragraph.

state->period != ddate->approx_period seems to be used to compare the
results of the previous and next two times.

I'm unsure what to do if I replace the conditional expression with
something else.

In addition, I don't understand the meaning of this.
"if state->period > ddata->real_period that's fine, and the driver
should continue"

At present, my new version of the implementation has passed the test
of the pwm_apply_state_debug() function.

Would you suggest I send the new implementation version before
continuing the discussion?

Thank you again for everything you’ve done.

>  - frac = DIV64_U64_ROUND_CLOSEST(num, state->period);
>    is wrong for two reasons:
>    it should round down and use the real period.
>
> Best regards
> Uwe
>
> --
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | https://www.pengutronix.de/ |