Re: [Intel-gfx] [PATCH 2/7] drm/i915/hwmon: Add HWMON current voltage support

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Tue, 13 Sep 2022 01:11:57 -0700, Gupta, Anshuman wrote:
>

Hi Anshuman,

> > -----Original Message-----
> > From: Dixit, Ashutosh <ashutosh.dixit@intel.com>
> > Sent: Monday, September 12, 2022 10:08 PM
> > To: Gupta, Anshuman <anshuman.gupta@intel.com>
> > Cc: Nilawar, Badal <badal.nilawar@intel.com>; intel-gfx@lists.freedesktop.org;
> > Tauro, Riana <riana.tauro@intel.com>; Ewins, Jon <jon.ewins@intel.com>;
> > linux-hwmon@vger.kernel.org
> > Subject: Re: [PATCH 2/7] drm/i915/hwmon: Add HWMON current voltage
> > support
> >
> > On Mon, 12 Sep 2022 07:09:28 -0700, Gupta, Anshuman wrote:
> > >
> > > > +static int
> > > > +hwm_in_read(struct hwm_drvdata *ddat, u32 attr, long *val) {
> > > > +	struct i915_hwmon *hwmon = ddat->hwmon;
> > > > +	intel_wakeref_t wakeref;
> > > > +	u32 reg_value;
> > > > +
> > > > +	switch (attr) {
> > > > +	case hwmon_in_input:
> > >
> > > Other attributes in this series take hwmon->lock before accessing i915
> > > registers , So do we need lock here as well ?
> >
> > The lock is being taken only for RMW and for making sure energy counter
> > updates happen atomically. We are not taking the lock for just reads so IMO no
> > lock is needed here.
>
> If that is the case, then why it needs to grab a lock for rmw on
> different Register ? Like in patch 3 of this series grabs
> hwmon->howmon_lock for rmw on two different register pkg_power_sku_unit
> and pkg_rapl_limit.

I don't see this happening, where do you see it?

> One register rmw should be independent of other register here ?

Yes each register RMW is independent. In Patch 3 only hwm_power_write (not
hwm_power_read) is taking the lock for RMW on pkg_rapl_limit (lock is not
taken for pkg_power_sku_unit). So the assumption is that RMW of a single
register are not atomic and must be serialized with a lock. Reads are
considered to not need a lock.

Thanks.
--
Ashutosh


> >
> > > > +		with_intel_runtime_pm(ddat->uncore->rpm, wakeref)
> > > > +			reg_value = intel_uncore_read(ddat->uncore, hwmon-
> > > > >rg.gt_perf_status);
> > > > +		/* In units of 2.5 millivolt */
> > > > +		*val =
> > > > DIV_ROUND_CLOSEST(REG_FIELD_GET(GEN12_VOLTAGE_MASK, reg_value)
> > * 25,
> > > > 10);
> > > > +		return 0;
> > > > +	default:
> > > > +		return -EOPNOTSUPP;
> > > > +	}
> > > > +}
> >
> > Thanks.
> > --
> > Ashutosh