[PATCH] mm: Work around Intel SNB GTT bug with some physical pages.

[Dave Airlie]

On Tue, May 8, 2012 at 12:13 AM, Stéphane Marchesin
<marcheu at chromium.org> wrote:
> While investing some Sandy Bridge rendering corruption, I found out
> that all physical memory pages below 1MiB were returning garbage when
> read through the GTT. This has been causing graphics corruption (when
> it's used for textures, render targets and pixmaps) and GPU hangups
> (when it's used for GPU batch buffers).
>
> I talked with some people at Intel and they confirmed my findings,
> and said that a couple of other random pages were also affected.
>
> We could fix this problem by adding an e820 region preventing the
> memory below 1 MiB to be used, but that prevents at least my machine
> from booting. One could think that we should be able to fix it in
> i915, but since the allocation is done by the backing shmem this is
> not possible.
>
> In the end, I came up with the ugly workaround of just leaking the
> offending pages in shmem.c. I do realize it's truly ugly, but I'm
> looking for a fix to the existing code, and am wondering if people on
> this list have a better idea, short of rewriting i915_gem.c to
> allocate its own pages directly.

Ouch, can Intel get some details on why these pages are "special" and
if they are special across chipsets, Ironlake? Ivybridge?

Like I can handle the < 1MB but the other selected pages look pretty
random or misc, (2005, 2011, 2013? years?, 40004000, some shout out to
the 4004?

Dave.