[Intel-gfx] [PATCH 2/5] drm/i915: Add per ctx batchbuffer wa for timestamp

[Chris Wilson]

Quoting Mika Kuoppala (2020-04-21 14:16:30)
> Restoration of a previous timestamp can collide
> with updating the timestamp, causing a value corruption.
> 
> Combat this issue by using indirect ctx bb to
> modify the context image during restoring process.
> 
> For render engine, we can preload value into
> scratch register. From which we then do the actual
> write with LRR. LRR is faster and thus less error prone.
> For other engines, no scratch is available so we
> must do a more complex sequence of sync and async LRMs.
> As the LRM is slower, the probablity of racy write
> raises and thus we still see corruption sometimes.
> 
> v2: tidying (Chris)
> 
> References: HSDES#16010904313
> Testcase: igt/i915_selftest/gt_lrc
> Suggested-by: Joseph Koston <joseph.koston at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Signed-off-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>

It's antagonising checkpatch for no good reason.
Acked-by: Chris Wilson <chris at chris-wilson.co.uk>

The only thing we have no feeling for is the impact of using this
mechanism on context switch times.
-Chris