Chris Wilson noticed that since

commit 1f83fee0 [v3.9]
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Nov 15 17:17:22 2012 +0100

    drm/i915: clear up wedged transitions

X can again get -EIO when it does not expect it. And even worse score
a SIGBUS when accessing gtt mmaps. The established ABI is that we
_only_ return an -EIO from execbuf - all other ioctls should just
work. And since the reset code moves all bos out of gpu domains and
clears out all the last_seqno/ring tracking there really shouldn't be
any reason for non-execbuf code to ever touch the hw and see an -EIO.

After some extensive discussions we've noticed that these spurios -EIO
are caused by i915_gem_wait_for_error:

http://www.mail-archive.com/intel-gfx@lists.freedesktop.org/msg20540.html

That is easy to fix by returning 0 instead of -EIO, since grabbing the
dev->struct_mutex does not yet mean that we actually want to touch the
hw. And so there is no reason at all to fail with -EIO.

But that's not the entire since, since often (at least it's easily
googleable) dmesg indicates that the reset fails and we declare the
gpu wedged. Then, quite a bit later X wakes up with the "Timed out
waiting for the gpu reset to complete" DRM_ERROR message in
wait_for_errror and brings down the desktop with an -EIO/SIGBUS.

So clearly we're missing a wakeup somewhere, since the gpu reset just
doesn't take 10 seconds to complete. And indeed we're do handle the
terminally wedged state wrong.

Fix this all up.

References: https://bugs.freedesktop.org/show_bug.cgi?id=63921
References: https://bugs.freedesktop.org/show_bug.cgi?id=64073
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>