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

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Message-ID: <ZroTN3UudwvIJ7oR@ashyti-mobl2.lan>
Date: Mon, 12 Aug 2024 15:50:47 +0200
From: Andi Shyti <andi.shyti@...ux.intel.com>
To: Raag Jadav <raag.jadav@...el.com>
Cc: jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
	rodrigo.vivi@...el.com, tursulin@...ulin.net, airlied@...il.com,
	daniel@...ll.ch, linux@...ck-us.net, andi.shyti@...ux.intel.com,
	andriy.shevchenko@...ux.intel.com, intel-gfx@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org, linux-hwmon@...r.kernel.org,
	linux-kernel@...r.kernel.org, anshuman.gupta@...el.com,
	badal.nilawar@...el.com, riana.tauro@...el.com,
	ashutosh.dixit@...el.com, karthik.poosa@...el.com
Subject: Re: [PATCH v5] drm/i915/hwmon: expose fan speed

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?

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?

Thanks,
Andi

> +	/*
> +	 * 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;
> +}