[Intel-gfx] [PATCH v2 1/2] drm/i915/bxt: work around HW coherency issue when accessing GPU seqno

[Chris Wilson]

On Fri, Aug 14, 2015 at 03:38:56PM +0300, Imre Deak wrote:
> By running igt/store_dword_loop_render on BXT we can hit a coherency
> problem where the seqno written at GPU command completion time is not
> seen by the CPU. This results in __i915_wait_request seeing the stale
> seqno and not completing the request (not considering the lost
> interrupt/GPU reset mechanism). I also verified that this isn't a case
> of a lost interrupt, or that the command didn't complete somehow: when
> the coherency issue occured I read the seqno via an uncached GTT mapping
> too. While the cached version of the seqno still showed the stale value
> the one read via the uncached mapping was the correct one.
> 
> Work around this issue by clflushing the corresponding CPU cacheline
> following any store of the seqno and preceding any reading of it. When
> reading it do this only when the caller expects a coherent view.
> 
> v2:
> - fix using the proper logical && instead of a bitwise & (Jani, Mika)
> - limit the workaround to A stepping, on later steppings this HW issue
>   is fixed

We have vfuncs in order to avoid the pointer dance (and boy is it a
pretty and quite convoluted dance).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre