[Mesa-dev] [PATCH] i965: Use tex_mocs instead of rb_mocs for GL images.

Kenneth Graunke kenneth at whitecape.org
Tue Jul 19 06:05:25 UTC 2016

On Monday, July 18, 2016 10:58:31 PM PDT Ben Widawsky wrote:
> On Mon, Jul 18, 2016 at 07:08:46PM -0700, Kenneth Graunke wrote:
> > Fixes a 10-20% performance regression in OglCSDof caused by commit
> > 5a8c89038abab0184ea72664ab390ec6ca58b4d6, which made images (in the
> > image load/store sense) use BDW_MOCS_PTE instead of BDW_MOCS_WB.
> > 
> > This seems sketchy, as the default PTE value is supposed to be
> > WB LLC eLLC, which is the same as our MOCS WB setting.  It's only
> > supposed to change when using a surface for display, which won't
> > ever happen for images.  Something may be wrong in the kernel...
> Which platform was the regression on? Broadwell has some weirdness if you
> disable PPGTT which might cause PTE MOCS to fall back to UC. I think at least
> aliasing PPGTT has been enabled since pretty early for Broadwell. If there is no
> ppgtt overrides here, then I'm not sure what would be going on - it sound
> sketchy to me too.

My Broadwell GT2 laptop running drm-intel-nightly from
2016y-05m-27d-12h-32m-45s (4.6.0 based), and Mark's Broadwell GT3e
boxes in Jenkins.

I tried to verify the kernel PTE settings, but I got totally lost
by the _PAGE_PAT stuff.  Earlier platforms I can follow easily...

This patch fixes the observed problem, but I still would like to
understand why the PTE isn't good enough...for renderbuffers, we
use the "Use PTE" setting...and could be missing out on some
performance there...
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