Re: [PATCH v5] drm/i915/hwmon: expose fan speed

Andi Shyti
2024-08-21T14:58:38Z

> > > + /*
> > > + * HW register value is accumulated count of pulses from
> > > + * PWM fan with the scale of 2 pulses per rotation.
> > > + */
> > > + rotations = pulses / 2;
> > > +
> > > + time = jiffies_delta_to_msecs(time_now - fi->time_prev);
> > > + if (unlikely(!time)) {
> > > + ret = -EAGAIN;
> > > + goto exit;
> > > + }
> >
> > Can you please add a comment describing how you obtain the speed
> > calculation?
>
> That's what I initially tried but ended up dropping it in favour of RPM
> formula below, which I found to be doing a better job of explaining than
> a few lines of description.
>
> > Basically at every read you store the values. Is it possible that
> > we don't have reads for a long time and the register resets more
> > than once?
>
> Considering a fan continuously running at higher speeds (for example 4000 RPM
> which is quite optimistic), with the scale of 2 pulses per rotation, a 32 bit
> register will take around a year to overflow, which is more than most usecases
> I could think of.

Which can be considered as a worse case scenario. I would have
preferred here a runtime calculation, which means read now, wait
a bit, read again and calculate. The read might be slow, but
efficient.

Anyway, your argument makes sense, so that I'm not going to push
on this, I already r-b'ed it.

Thanks,
Andi