[Intel-gfx] [PATCH 13/21] drm/i915: add support for 64K scratch page

[Chris Wilson]

Quoting Matthew Auld (2017-09-22 18:32:44)
> Before we can fully enable 64K pages, we need to first support a 64K
> scratch page if we intend to support the case where we have object sizes
> < 2M, since any scratch PTE must also point to a 64K region.  Without
> this our 64K usage is limited to objects which completely fill the
> page-table, and therefore don't need any scratch.
> 
> v2: add reminder about why 48b PPGTT
> 
> Reported-by: Chris Wilson <chris at chris-wilson.co.uk>
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

We really, really should write protect the scratch page so that we can
share it between ppgtt. We can't make it simply unreadable as we use it
for padding.
-Chris