Re: [PATCH v4] hwmon: pwm-fan: Fix RPM calculation

[Guenter Roeck <linux@roeck-us.net>]

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@konsulko.com>
> ---
> 
>   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
>