[PATCH 1/2] drm/i915: Restore GT coarse power gating workaround

[Chris Wilson]

Quoting Imre Deak (2019-12-20 12:29:13)
> On Thu, Dec 19, 2019 at 04:42:44PM +0000, Chris Wilson wrote:
> > Quoting Imre Deak (2019-11-14 16:42:24)
> > > The workaround to disable coarse power gating is still needed on SKL
> > > GT3/GT4 machines and since the RC6 context corruption was discovered by
> > > the hardware team also on all GEN9 machines. Restore applying the
> > > workaround.
> > 
> > What exactly is the link between powergating and the rc6 power context
> > corruption? Disabling powergating entirely is quite a significant
> > regression -- and we can't partially enable powergating for idle engines
> > as the HW refuses to cooperate.
> > 
> > So is it safe to enable powergating if only vcs is busy [rcs is idle]?
> 
> The problem with CPG wrt. possible RC6 corruption is that if a
> corruption happens on one engine while another is already idle (having
> saved its context) then this idle engine won't be able to resume any
> more (since it cannot restore its context from the corrupted RC6 state).

Ah, but we always switch to a scratch kernel context on idle, and
that context has CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT set. Or it refers
to the power context instead, which is beyond our control?
-Chris