On Tue, Nov 10, 2020 at 05:54:34PM +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. If the width of the measurement interval is zero we
> don't update the RPM value to avoid dividing by zero.
>
> We also don't need to do 64-bit division, with 32-bits we can handle a
> fan running at over 4 million RPM.
>
> Signed-off-by: Paul Barker <pbarker@xxxxxxxxxxxx>
> ---
>
> Changes from v2:
>
> * Don't update the RPM value if delta=0.
>
> drivers/hwmon/pwm-fan.c | 13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
> index bdba2143021a..d1fd50c32514 100644
> --- a/drivers/hwmon/pwm-fan.c
> +++ b/drivers/hwmon/pwm-fan.c
> @@ -54,14 +54,15 @@ 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;
> + if (delta) {
> + pulses = atomic_read(&ctx->pulses);
> + atomic_sub(pulses, &ctx->pulses);
> + ctx->rpm = (unsigned int)(pulses * 1000 * 60) /
> + (ctx->pulses_per_revolution * delta);
> + }
>
> ctx->sample_start = ktime_get();
I know I am a bit nitpicking here, but should ctx->sample_start only be updated
if delta != 0 ? We don't reset pulses if delta == 0, meaning the next reading
will be based on the old start time (even if the difference was less than 1 ms).
Thanks,
Guenter
> mod_timer(&ctx->rpm_timer, jiffies + HZ);
>
> base-commit: f8394f232b1eab649ce2df5c5f15b0e528c92091
> --
> 2.29.2
>
