[Intel-gfx] [PATCH 16/20] drm/i915/gtt: Fill scratch page

[Mika Kuoppala]

During review of dynamic page tables series, I was able
to hit a lite restore bug with execlists. I assume that
due to incorrect pd, the batch run out of legit address space
and into the scratch page area. The ACTHD was increasing
due to scratch being all zeroes (MI_NOOPs). And as gen8
address space is quite large, the hangcheck happily waited
for a long long time, keeping the process effectively stuck.

According to Chris Wilson any modern gpu will grind to halt
if it encounters commands of all ones. This seemed to do the
trick and hang was declared promptly when the gpu wandered into
the scratch land.

Signed-off-by: Mika Kuoppala <mika.kuoppala at intel.com>
---
 drivers/gpu/drm/i915/i915_gem_gtt.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index ccdb35f..26d1d45 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -2153,6 +2153,7 @@ static int alloc_scratch_page(struct i915_address_space *vm)
 		return ret;
 	}
 
+	fill_px(vm->dev, sp, ~0ULL);
 	set_pages_uc(px_page(sp), 1);
 
 	vm->scratch_page = sp;
-- 
1.9.1