[Intel-gfx] [PATCH 12/18] drm/i915/gtt: Fill scratch page

[Chris Wilson]

On Thu, Jun 25, 2015 at 06:35:14PM +0300, Mika Kuoppala wrote:
> 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)

Thinking about this, could we add a scratch page checker to hangcheck?
Just check the first/last u64 perhaps? Or random offset_in_page?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre