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

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Message-ID: <ZrsY6tMts81T-uFa@smile.fi.intel.com>
Date: Tue, 13 Aug 2024 11:27:22 +0300
From: Andy Shevchenko <andriy.shevchenko@...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,
	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

On Tue, Aug 13, 2024 at 08:45:19AM +0300, Raag Jadav wrote:
> On Mon, Aug 12, 2024 at 04:15:14PM +0300, Andy Shevchenko wrote:
> > On Mon, Aug 12, 2024 at 01:45:38PM +0530, Raag Jadav wrote:

...

> > > +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;
> > 
> > Isn't it the abs_diff() reimplementation?
> 
> Not exactly. This is specific to 32 bit register overflow, so we count
> from max value.

I see. But since you have the both variables of u32, why:
1) UINT_MAX?
2) Not simply using

	pulses = reg_val - fi->reg_val_prev;

which will wrap over correctly?

Note, in your case (in comparison to the wrap over variant) the off-by-one is
present. Is it on purpose?

-- 
With Best Regards,
Andy Shevchenko