On Sun, 8 Nov 2020 at 03:47, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>
> On Thu, Nov 05, 2020 at 04:41:46PM +0000, Paul Barker wrote:
> > To convert the number of pulses counted into an RPM estimation, we need
> > to divide by the width of our measurement interval instead of
> > multiplying by it.
> >
> > We also don't need to do 64-bit division, with 32-bits we can handle a
> > fan running at over 4 million RPM.
> >
> > As the sample_timer() function is called once per second, delta will usually
> > be just over 1000 and should never be zero, avoiding the risk of a divide by
> > zero exception.
> >
> > Signed-off-by: Paul Barker <pbarker@xxxxxxxxxxxx>
> > ---
> > drivers/hwmon/pwm-fan.c | 7 +++----
> > 1 file changed, 3 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
> > index bdba2143021a..2f2950629892 100644
> > --- a/drivers/hwmon/pwm-fan.c
> > +++ b/drivers/hwmon/pwm-fan.c
> > @@ -54,14 +54,13 @@ static irqreturn_t pulse_handler(int irq, void *dev_id)
> > static void sample_timer(struct timer_list *t)
> > {
> > struct pwm_fan_ctx *ctx = from_timer(ctx, t, rpm_timer);
> > + unsigned int delta = ktime_ms_delta(ktime_get(), ctx->sample_start);
> > int pulses;
> > - u64 tmp;
> >
> > pulses = atomic_read(&ctx->pulses);
> > atomic_sub(pulses, &ctx->pulses);
> > - tmp = (u64)pulses * ktime_ms_delta(ktime_get(), ctx->sample_start) * 60;
> > - do_div(tmp, ctx->pulses_per_revolution * 1000);
> > - ctx->rpm = tmp;
> > + ctx->rpm = (unsigned int)(pulses * 1000 * 60) /
> > + (ctx->pulses_per_revolution * delta);
>
> delta can, at least in theory, be 0. While that is quite unlikely to happen,
> the code should handle this situation.
I'll send a v3 which bails out if delta=0.
Thanks,
--
Paul Barker
Konsulko Group
