linux-kernel - Re: [PATCH 1/2] pwm: meson: consider 128 a valid pre-divider

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Date:   Tue, 2 Apr 2019 21:22:55 +0200
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     thierry.reding@...il.com, linux-pwm@...r.kernel.org,
        linux-amlogic@...ts.infradead.org,
        Neil Armstrong <narmstrong@...libre.com>, jbrunet@...libre.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        bichao.zheng@...ogic.com
Subject: Re: [PATCH 1/2] pwm: meson: consider 128 a valid pre-divider

Hello Uwe,

On Mon, Apr 1, 2019 at 8:38 PM Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
>
> Hello Martin,
>
> On Mon, Apr 01, 2019 at 08:18:16PM +0200, Martin Blumenstingl wrote:
> > diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
> > index f6e738ad7bd9..4b708c1fcb1d 100644
> > --- a/drivers/pwm/pwm-meson.c
> > +++ b/drivers/pwm/pwm-meson.c
> > @@ -188,7 +188,7 @@ static int meson_pwm_calc(struct meson_pwm *meson,
> >       do_div(fin_ps, fin_freq);
> >
> >       /* Calc pre_div with the period */
> > -     for (pre_div = 0; pre_div < MISC_CLK_DIV_MASK; pre_div++) {
> > +     for (pre_div = 0; pre_div <= MISC_CLK_DIV_MASK; pre_div++) {
> >               cnt = DIV_ROUND_CLOSEST_ULL((u64)period * 1000,
> >                                           fin_ps * (pre_div + 1));
> >               dev_dbg(meson->chip.dev, "fin_ps=%llu pre_div=%u cnt=%u\n",
>
> You could even calculate pre_div without the loop.
>
> Something like:
>
>         u64 pre_div = (u64)period * rate;
>         do_div_round_up(pre_div, NSEC_PER_SEC * 0xffff);
>         pre_div--;
>
> (I didn't check rounding and maybe its off by one and ...) This would
> also get rid of the strange 1000 that is currently used in the
> calculation without a real benefit (unless I missed something).
personally I prefer using this simple patch applied first as it is
easy to review and (due to the Fixes tag) may get backported to stable
kernels.
I'm not saying I don't like your suggestion, I propose to postpone
implementing this cleanup. I need to have a closer look at the
calculation because three values are derived from the input clock rate
(pre_div, cnt, duty_cnt) and I don't want to mess up the cases that
are already working as of today.

Please let me know what you think.


Regards
Martin