[Intel-gfx] [RFC] drm/i915/lrc: allocate separate page for HWSP

[Chris Wilson]

Quoting Daniele Ceraolo Spurio (2017-07-06 17:10:40)
> On gen8+ we're currently using the PPHWSP of the kernel ctx as the
> global HWSP. However, when the kernel ctx gets submitted (e.g. from
> __intel_autoenable_gt_powersave) the HW will use that page as both
> HWSP and PPHWSP. Currently we're not seeing any problem because the
> conflict happens at offsets below 0x30 in an area we don't access,
> but that is not guaranteed to be true for future platform.
> 
> To avoid the conflict, instead of re-using the PPHWSP of the kernel
> ctx we can allocate a separate page for the HWSP like what happens for
> pre-execlists platform.

If there are no conflicts, then just skip the patch, and really there
shouldn't be since the writes we care about are ordered and the writes
we don't, we don't. We will need to move to per-context hwsp in the near
future which should alleviate these concerns.
-Chris