[Mesa-dev] [PATCH 7/7] i965: Drop non-LLC lunacy in the program cache code.

[Kenneth Graunke]

On Monday, July 24, 2017 3:54:11 AM PDT Emil Velikov wrote:
> Hi Ken,
> 
> Admittedly I'm not an expert in the area, so perhaps a rather silly question.
> 
> On 22 July 2017 at 00:17, Kenneth Graunke <kenneth at whitecape.org> wrote:
> 
> > +#ifdef USE_SSE41
> > +      if (!cache->bo->cache_coherent && cpu_has_sse4_1)
> > +         _mesa_streaming_load_memcpy(map, cache->map, cache->next_offset);
> > +      else
> > +#endif
> > +         memcpy(map, cache->map, cache->next_offset);
> The other user of _mesa_streaming_load_memcpy -
> intel_miptree_map/intel_miptree_map_movntdqa does not seem to check
> for the coherency flag.
> 
> Which makes me wonder:
> Did you intentionally combine the SSE4.1 check with the
> !cache_coherent one, should there be a similar check in the miptree
> code or the two cases are orthogonal?
> 
> Thanks
> Emil
> 

The other code uses brw->has_llc.  Basically, on LLC platforms, all
buffers other than scanout are coherent.  On non-LLC, almost all buffers
are non-coherent.  We originally didn't have a bo->cache_coherent flag,
and used brw->has_llc as the distinguishing factor.

On non-LLC, you can make buffers coherent by enabling snooping (but it's
expensive).  We haven't ever done that yet, though Chris has patches to
do so for query object buffers, where we want the CPU and GPU to be able
to read an "I'm done!" flag for CheckQuery.

So, it would probably be reasonable to change intel_miptree_map to use
bo->cache_coherent instead of brw->has_llc, though this is unlikely to
matter in practice since snooping for textures doesn't make much sense,
and on LLC systems, we're probably not going to map the scanout buffer.

MOVNTDQA gives faster streaming read performance when sourcing from
uncached memory, apparently.  Non-coherent BOs bypass the CPU caches,
so we want to use it there.


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