[Mesa-dev] [PATCH 4/7] i965: Disable access to CPU mmap for async access on non-LLC machines

[Kenneth Graunke]

From: Chris Wilson <chris at chris-wilson.co.uk>

If the user triggers an implicit batch flush while holding access to a
CPU mapped buffer, that mmapping will be invalidated by the kernel for
non-LLC devices. (The kernel when executing a batch will change the
cache domain of the buffers in that batch, which for non-LLC CPU access
will cause that buffer to be clflushed and any further CPU access to be
discarded.) To prevent this, simply disallow any CPU async mmap access.
The cases where async CPU access to a non-LLC buffer should continue to
be allowed via their preferred snooping path.

v2 (Ken): Reword the comment slightly.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
Cc: Matt Turner <mattst88 at gmail.com>
---
 src/mesa/drivers/dri/i965/brw_bufmgr.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_bufmgr.c b/src/mesa/drivers/dri/i965/brw_bufmgr.c
index 201bf9dc77f..74ed543eece 100644
--- a/src/mesa/drivers/dri/i965/brw_bufmgr.c
+++ b/src/mesa/drivers/dri/i965/brw_bufmgr.c
@@ -762,10 +762,18 @@ can_map_cpu(struct brw_bo *bo, unsigned flags)
    if (bo->cache_coherent)
       return true;
 
-   if (flags & MAP_PERSISTENT)
-      return false;
-
-   if (flags & MAP_COHERENT)
+   /* If PERSISTENT or COHERENT are set, the mmapping needs to remain valid
+    * across batch flushes where the kernel will change cache domains of the
+    * bo, invalidating continued access to the CPU mmap on non-LLC device.
+    *
+    * Similarly, ASYNC typically means that the buffer will be accessed via
+    * both the CPU and the GPU simultaneously.  Batches may be executed that
+    * use the BO even while it is mapped.  While OpenGL technically disallows
+    * most drawing while non-persistent mappings are active, we may still use
+    * the GPU for blits or other operations, causing batches to happen at
+    * inconvenient times.
+    */
+   if (flags & (MAP_PERSISTENT | MAP_COHERENT | MAP_ASYNC))
       return false;
 
    return !(flags & MAP_WRITE);
-- 
2.13.2