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

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Feb 28, 2023 at 01:18:55PM -0800, Dixit, Ashutosh wrote:
> 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?
> 

Nothing should ever be "arbitrary" but have some reason. Arbitrary is
if you glue all the possible attributes onto a wall and then select the
ones to use by throwing darts at it.

powerX_min, powerX_max and powerX_crit are typically hard limits which
can not actively be influenced without drastic measures such as turning
off some hardware. powerX_cap is supposed to be more flexible; the
assumption is that the hardware or firmware has some means to control power
such that it does not exceed powerX_cap (while maintaining operational
status), for example by modifying operational frequencies.

Nowadays everything may be a bit more flexible; for example, one could
imagine that a modern system could (via software) reduce the operational
frequency of the system if power consumption exceeds powerX_max or
powerX_crit. The distinction would be that with powerX_cap, the hardware
or firmware would in general be in control, while with powerX_max
and powerX_crit the host software would be in control.

> 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.

powerX_min is supported by the infrastructure. It not being documented
is an oversight.

Guenter

> 
> 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