[Intel-gfx] [PATCH 15/21] 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.

v2: Use 0xffff00ff pattern (Chris)

Cc: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala at intel.com>
---
 drivers/gpu/drm/i915/i915_gem_gtt.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index 43fa543..a2a0c88 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -2168,6 +2168,8 @@ void i915_global_gtt_cleanup(struct drm_device *dev)
 	vm->cleanup(vm);
 }
 
+#define SCRATCH_PAGE_MAGIC 0xffff00ffffff00ffULL
+
 static int alloc_scratch_page(struct i915_address_space *vm)
 {
 	struct i915_page_scratch *sp;
@@ -2185,6 +2187,7 @@ static int alloc_scratch_page(struct i915_address_space *vm)
 		return ret;
 	}
 
+	fill_px(vm->dev, sp, SCRATCH_PAGE_MAGIC);
 	set_pages_uc(px_page(sp), 1);
 
 	vm->scratch_page = sp;
-- 
1.9.1