drm/i915: Enforce write ordering through the GTT
We need to ensure that writes through the GTT land before any
modification to the MMIO registers and so must impose a mandatory write
barrier when flushing the GTT domain. This was revealed by relaxing the
write ordering by experimentally mapping the registers and the GATT as
write-combining.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index c79c0b6..f9c093c 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2393,6 +2393,12 @@
obj->last_fenced_ring = NULL;
}
+ /* Ensure that all CPU reads are completed before installing a fence
+ * and all writes before removing the fence.
+ */
+ if (obj->base.read_domains & I915_GEM_DOMAIN_GTT)
+ mb();
+
return 0;
}
@@ -2833,10 +2839,16 @@
if (obj->base.write_domain != I915_GEM_DOMAIN_GTT)
return;
- /* No actual flushing is required for the GTT write domain. Writes
+ /* No actual flushing is required for the GTT write domain. Writes
* to it immediately go to main memory as far as we know, so there's
* no chipset flush. It also doesn't land in render cache.
+ *
+ * However, we do have to enforce the order so that all writes through
+ * the GTT land before any writes to the device, such as updates to
+ * the GATT itself.
*/
+ wmb();
+
i915_gem_release_mmap(obj);
old_write_domain = obj->base.write_domain;