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

[Raag Jadav <raag.jadav@intel.com>]

On Mon, Aug 12, 2024 at 03:50:47PM +0200, Andi Shyti wrote:
> Hi Raag,
> 
> > +static int
> > +hwm_fan_read(struct hwm_drvdata *ddat, u32 attr, long *val)
> > +{
> > +	struct i915_hwmon *hwmon = ddat->hwmon;
> > +	struct hwm_fan_info *fi = &ddat->fi;
> > +	u64 rotations, time_now, time;
> > +	intel_wakeref_t wakeref;
> > +	u32 reg_val, pulses;
> > +	int ret = 0;
> > +
> > +	if (attr != hwmon_fan_input)
> > +		return -EOPNOTSUPP;
> > +
> > +	wakeref = intel_runtime_pm_get(ddat->uncore->rpm);
> > +	mutex_lock(&hwmon->hwmon_lock);
> > +
> > +	reg_val = intel_uncore_read(ddat->uncore, hwmon->rg.fan_speed);
> > +	time_now = get_jiffies_64();
> > +
> > +	/* Handle HW register overflow */
> > +	if (reg_val >= fi->reg_val_prev)
> > +		pulses = reg_val - fi->reg_val_prev;
> > +	else
> > +		pulses = UINT_MAX - fi->reg_val_prev + reg_val;
> > +
> > +	/*
> > +	 * 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.

Raag

> > +	/*
> > +	 * Convert to minutes for calculating RPM.
> > +	 * RPM = number of rotations * msecs per minute / time in msecs
> > +	 */
> > +	*val = DIV_ROUND_UP(rotations * (MSEC_PER_SEC * 60), time);
> > +
> > +	fi->reg_val_prev = reg_val;
> > +	fi->time_prev = time_now;
> > +exit:
> > +	mutex_unlock(&hwmon->hwmon_lock);
> > +	intel_runtime_pm_put(ddat->uncore->rpm, wakeref);
> > +	return ret;
> > +}