[PATCH] drm/i915: Access to snooped system memory through the GTT is incoherent

[PATCH] drm/i915: Access to snooped system memory through the GTT is incoherent

[Chris Wilson]

We ignore all the user requests to handle flushing to the GTT domain if
the user requests such on a snoopable bo, and as such access through the
GTT to such pages remains incoherent. The specs even warn that such
behaviour is undefined - a strong reason never to do so.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 65cf019..6438b2c 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1417,6 +1417,12 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 
 	trace_i915_gem_object_fault(obj, page_offset, true, write);
 
+	/* Access to snoopable pages through the GTT is incoherent. */
+	if (obj->cache_level != I915_CACHE_NONE && !HAS_LLC(dev)) {
+		ret = -EINVAL;
+		goto unlock;
+	}
+
 	/* Now bind it into the GTT if needed */
 	ret = i915_gem_object_pin(obj, 0, I915_PIN_MAPPABLE);
 	if (ret)
-- 
1.7.10.4

Re: [PATCH] drm/i915: Access to snooped system memory through the GTT is incoherent

[Daniel Vetter]

On Sun, Dec 16, 2012 at 12:43:36PM +0000, Chris Wilson wrote:
> We ignore all the user requests to handle flushing to the GTT domain if
> the user requests such on a snoopable bo, and as such access through the
> GTT to such pages remains incoherent. The specs even warn that such
> behaviour is undefined - a strong reason never to do so.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

Queued for -next, thanks for the patch.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch