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

[Nylon Chen]

Hi Uwe

I'm glad the example was helpful to you. I will double-check and
verify my implementation once again.

Thank you for your assistance.

Uwe Kleine-König <u.kleine-koenig at pengutronix.de> 於 2023年9月8日 週五 下午10:50寫道：
>
> Hello,
>
> On Fri, Sep 08, 2023 at 06:41:00PM +0800, Nylon Chen wrote:
> > 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.
>
> There are two things to consider:
>
>  - usually the hardware doesn't support all requestable states because
>    the hardware's quantum is > 1 ns. That is, it might for example
>    support periods in the form (160 ns * i / 3) for i in 1 .. 1023.
>
>    If this hardware runs with i = 500 (that is period ~= 26666.66
>    ns) because the first channel is configured to run with period =
>    26667, and .request is called for the 2nd channel with .period =
>    26700 ns, there is no need to refuse that, because 26666.66 is the
>    best possible approximation for 26700 ns anyhow.
>
>  - .apply is supposed to implement the highest possible period that
>    isn't bigger than the requested period. So in the above case even if
>    the hardware runs at 26666.66 ns without the possibility to change
>    that, a request to configure for period = 30000 ns could succeed (and
>    keep 26666.66 ns).
>
> > Would you suggest I send the new implementation version before
> > continuing the discussion?
>
> Note that the above implements the optimal behaviour for a driver.
> (For some definition of "optimal" that admittedly also yields strange
> behaviour at times. The reasoning for this to the be thing to implement
> is, that's the corner cases are easier to implement, idempotency is
> possible and it's easier to work with than rounding to the nearest
> possible value.)
>
> While I'd like to see the sifive driver to implement this optimal
> behaviour, it's not mandatory that you convert the driver to that
> behaviour. Just make sure to not make it worse.
>
> So to answer your question: If you understood what I wrote above and are
> motivated to improve the driver, it would be great to do that before the
> next review round.
>
> Best regards
> Uwe
>
> --
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | https://www.pengutronix.de/ |