[Intel-gfx] [PATCH] drm/i915/byt: Avoid tweaking evaluation thresholds

[Chris Wilson]

On Wed, Jan 25, 2017 at 02:31:08PM +0200, Mika Kuoppala wrote:
> Certain Baytrails, namely the 4 cpu core variants, have been
> plaqued by spurious system hangs, mostly occurring with light loads.
> 
> Multiple bisects by various people point to a commit which changes the
> reclocking strategy for Baytrail to follow its bigger brethen:
> commit 8fb55197e64d ("drm/i915: Agressive downclocking on Baytrail")
> 
> There is also a review comment attached to this commit from Deepak S
> on avoiding punit access on Cherryview and thus it is excluded on
> common reclocking path. By taking the same approach and omitting
> the punit access by not tweaking the thresholds when the hardware
> has been asked to move into different frequency, considerable gains
> in stability have been observed.
> 
> With J1900 box, light render/video load would end up in system hang
> in usually less than 12 hours. With this patch applied, the cumulative
> uptime has now been 34 days without issues. To provoke system hang,
> light loads on both render and bsd engines in parallel have been used:
> glxgears >/dev/null 2>/dev/null &
> mpv --vo=vaapi --hwdec=vaapi --loop=inf vid.mp4
> 
> So far, author has not witnessed system hang with above load
> and this patch applied. Reports from the tenacious people at
> kernel bugzilla are also promising.
> 
> Considering that the punit access frequency with this patch is
> considerably less, there is a possibility that this will push
> the, still unknown, root cause past the triggering point on most loads.
> Further work on investigating the punit accesses on byt is welcomed.

Please find the underlying problem and not disabling rps for all vlv
for a GT specific problem.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre