[PATCH] drm/i915/hwmon: Use 0 to designate disabled PL1 power limit

[Dixit, Ashutosh]

On Tue, 28 Mar 2023 16:35:43 -0700, Ashutosh Dixit wrote:
>
> On ATSM the PL1 limit is disabled at power up. The previous uapi assumed
> that the PL1 limit is always enabled and therefore did not have a notion of
> a disabled PL1 limit. This results in erroneous PL1 limit values when the
> PL1 limit is disabled. For example at power up, the disabled ATSM PL1 limit
> was previously shown as 0 which means a low PL1 limit whereas the limit
> being disabled actually implies a high effective PL1 limit value.
>
> To get round this problem, the PL1 limit uapi is expanded to include a
> special value 0 to designate a disabled PL1 limit.

This patch is another attempt to show when the PL1 power limit is disabled
and to disable it when it needs to. Previous abandoned attempts to do this
are [1] and [2].

The preferred way to do this was [2] but that was NAK'd by hwmon folks (see
[2]). That is why here we fall back on the approach in [1].

This patch is identical to [1] except that the value used to disable the
PL1 limit has been changed to 0 (from -1 in [1]) as was suggested in [2]
(both -1 and 0 seem ok for the purpose).

> Bug: https://gitlab.freedesktop.org/drm/intel/-/issues/8062
> Bug: https://gitlab.freedesktop.org/drm/intel/-/issues/8060

The link between this patch and these pretty serious bugs might not be
immediately clear so here's an explanation:

* Because on ATSM the PL1 power limit is disabled on power up and there
  were no means to enable it, in 6fd3d8bf89fc we implemented the means to
  enable the limit when the PL1 hwmon entry (power1_max) was written to.

* Now there is an IGT igt at i915_hwmon@hwmon_write which (a) reads orig value
  from all hwmon sysfs  (b) does a bunch of random writes and finally (c)
  restores the orig value read. On ATSM since the orig value was 0, when
  the IGT restores the 0 value, the PL1 limit is now enabled with a value
  of 0.

* PL1 limit of 0 implies a low PL1 limit which causes GPU freq to fall to
  100 MHz. This causes GuC FW load and several IGT's to start timing out
  and gives rise the above (and even more) bugs about GuC FW load timing
  out.

* After this patch, writing 0 would disable the PL1 limit instead of
  enabling it, avoiding the freq drop issue above, and resolving this Intel
  CI issue.

Thanks.
--
Ashutosh

[1] https://patchwork.freedesktop.org/patch/522612/?series=113972&rev=1
[2] https://patchwork.freedesktop.org/patch/522652/?series=113984&rev=1