Re: [PATCH 3/7] drm/i915/hwmon: Power PL1 limit and TDP setting

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

On Fri, 12 Aug 2022 11:06:58 -0700, Guenter Roeck wrote:
>

Hi Guenter/linux-hwmon,


> On 8/12/22 10:37, Badal Nilawar wrote:
> > From: Dale B Stimson <dale.b.stimson@intel.com>
> >
> > Use i915 HWMON to display/modify dGfx power PL1 limit and TDP setting.
> >

/snip/

>
> Acked-by: Guenter Roeck <linux@roeck-us.net>
>
> > ---
> >   .../ABI/testing/sysfs-driver-intel-i915-hwmon |  20 ++
> >   drivers/gpu/drm/i915/i915_hwmon.c             | 176 +++++++++++++++++-
> >   drivers/gpu/drm/i915/i915_reg.h               |  16 ++
> >   drivers/gpu/drm/i915/intel_mchbar_regs.h      |   7 +
> >   4 files changed, 217 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/ABI/testing/sysfs-driver-intel-i915-hwmon b/Documentation/ABI/testing/sysfs-driver-intel-i915-hwmon
> > index 24c4b7477d51..9a2d10edfce8 100644
> > --- a/Documentation/ABI/testing/sysfs-driver-intel-i915-hwmon
> > +++ b/Documentation/ABI/testing/sysfs-driver-intel-i915-hwmon
> > @@ -5,3 +5,23 @@ Contact:	dri-devel@lists.freedesktop.org
> >   Description:	RO. Current Voltage in millivolt.
> >			Only supported for particular Intel i915 graphics
> > platforms.
> > +
> > +What:		/sys/devices/.../hwmon/hwmon<i>/power1_max
> > +Date:		June 2022
> > +KernelVersion:	5.19
> > +Contact:	dri-devel@lists.freedesktop.org
> > +Description:	RW. Card reactive sustained  (PL1/Tau) power limit in microwatts.
> > +
> > +		The power controller will throttle the operating frequency
> > +		if the power averaged over a window (typically seconds)
> > +		exceeds this limit.

We exposed this as 'power1_max' previously. However this is a "power
limit".

https://github.com/torvalds/linux/blob/master/Documentation/hwmon/sysfs-interface.rst

says power1_max is "Maximum power". On the other hand, power1_cap is "If
power use rises above this limit, the system should take action to reduce
power use". So it would seem we should have chosen power1_cap for this
power limit instead of power1_max? So do you think we should change this to
power1_cap instead? Though even power1_max has an associated alarm so it
also seems to be a sort of limit.

Is there any guidance as to how these different power limits should be
used? Generally speaking is: power1_max <= power1_cap <= power1_crit, or is
it arbitrary or something else?

Also, only power1_cap seems to have power1_cap_min and power1_cap_max (in
case we wanted to use min/max values for the limits), not the others.

Separately, we have already used up power1_crit (which is the other limit
in official hwmon power limits) so we can't use that.

Thanks.
--
Ashutosh