[Intel-gfx] [PATCH 2/2] drm/i915: Disable snooping (userptr, set-cache-level) on gen4

[Chris Wilson]

Quoting Ville Syrjälä (2017-09-05 20:01:03)
> On Tue, Sep 05, 2017 at 02:47:35PM +0100, Chris Wilson wrote:
> > The original gen4 has an issue where writes (both render and blt) into
> > snoopable pages are lost.
> 
> BTW is this documented somewhere, or just folk lore? I only found bspec
> notes that snooped->snooped and tiled+snooped isn't allowed. But I
> guess this is something more generic?

Just experience indeed. Never stumbled across anything to actually
explain it, especially why it is limited to just a couple of machines.
You would have expected it to have at least made the cspec errata list,
if it was a real issue! But having stumbled across the same issue again
with mesa, I don't think it is just the way I implemented the blits.
-Chris