[Intel-gfx] [PATCH] drm/i915: Force uncached PPAT for debugging purposes.

[Vivi, Rodrigo]

On Tue, 2017-02-28 at 09:05 +0000, Chris Wilson wrote:
> On Mon, Feb 27, 2017 at 05:18:08PM -0800, Ben Widawsky wrote:
> > On 17-02-27 17:12:07, Rodrigo Vivi wrote:
> > >Many screen corruptions and hangs in the past were somehow
> > >related to the caches. In many situations forcing the uncached
> > >was useful at least to narrow down the issue by confirming it
> > >was cache related.
> > >
> > >Instead of having to hardcode it everytime that we suspect on
> > >this table let's provide a mechanism to disable these
> > >cache leves on this private table (PPAT).
> > >
> > >Cc: Ben Widawsky <benjamin.widawsky at intel.com>
> > >Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> > >Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
> > 
> > I think this is a cool idea, it could equally be achieved by modifying the PTE
> > encoding function. In my head, modifying the pte encode makes more sense since
> > it applies to all legacy context, advanced context, and ggtt entries.
> 
> Changing the mocs table is not the same as changing the PTE. Both are
> useful for different types of bugs.

Indeed.

Although I believe Ben mentioned the pat_sel inside PTE. But if this is
the case I checked here and it seems that it is spread in different
functions here so I believe the simplest and safiest place to do is here
inside the PPAT setup.

> -Chris
>