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

[Imre Deak]

On pe, 2015-08-14 at 14:12 +0100, Chris Wilson wrote:
> 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).

Ok, I'll add new get_seqno/set_seqno vfuncs.

> -Chris
>