Re: [PATCH v2 3/3] pwm: meson: improve pwm calculation precision.

From: Jerome Brunet
Date: Fri Jul 07 2017 - 04:58:32 EST


On Thu, 2017-07-06 at 23:21 +0200, Thierry Reding wrote:
> On Thu, Jun 08, 2017 at 02:24:16PM +0200, Jerome Brunet wrote:
> > When using input clocks with high rates, such as clk81 (166MHz), the
> > fin_ns = NSEC_PER_SEC / fin_freq can introduce a significant error.
> >
> > Ex: fin_freq = 166666667, NSEC_PER_SEC = 1000000000
> > ÂÂÂÂfin_ns = 5,9999999
> >
> > which is, of course, rounded down to 5. This introduce an error of ~20%
> > on the period requested from the pwm.
> >
> > This patch use ps instead of ns (and 64bits integer) to perform the
> > calculation. This should give a good enough precision.
> >
> > Fixes: 211ed630753d ("pwm: Add support for Meson PWM Controller")
> > Signed-off-by: Jerome Brunet <jbrunet@xxxxxxxxxxxx>
> > ---
> > Âdrivers/pwm/pwm-meson.c | 15 +++++++++------
> > Â1 file changed, 9 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
> > index b911a944744a..4cdc66f7f718 100644
> > --- a/drivers/pwm/pwm-meson.c
> > +++ b/drivers/pwm/pwm-meson.c
> > @@ -163,7 +163,8 @@ static int meson_pwm_calc(struct meson_pwm *meson,
> > Â ÂÂunsigned int duty, unsigned int period)
> > Â{
> > Â unsigned int pre_div, cnt, duty_cnt;
> > - unsigned long fin_freq = -1, fin_ns;
> > + unsigned long fin_freq = -1;
> > + u64 fin_ps;
> > Â
> > Â if (~(meson->inverter_mask >> id) & 0x1)
> > Â duty = period - duty;
> > @@ -179,13 +180,14 @@ static int meson_pwm_calc(struct meson_pwm *meson,
> > Â }
> > Â
> > Â dev_dbg(meson->chip.dev, "fin_freq: %lu Hz\n", fin_freq);
> > - fin_ns = NSEC_PER_SEC / fin_freq;
> > + fin_ps = ((u64)NSEC_PER_SEC * 1000) / fin_freq;
>
> This failed to build, so I had to change this division to a do_div().

Oh ! Indeed :(
I have mostly tested with arm64 which is fine.
I thought I had tested arm(32) build as well but I just noticed that MESON_PWM
option is disabled in multi_v7_defconfig.

Thanks for catching and fixing this Thierry !


>
> Thierry