drm/ttm: Optimize reservation slightly

Reservation locking currently always takes place under the LRU spinlock.
Hence, strictly there is no need for an atomic_cmpxchg call; we can use
atomic_read followed by atomic_write since nobody else will ever reserve
without the lru spinlock held.
At least on Intel this should remove a locked bus cycle on successful
reserve.

Note that thit commit may be obsoleted by the cross-device reservation work.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index d1e5326d442c..5f61f133b419 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -220,7 +220,7 @@ int ttm_bo_reserve_locked(struct ttm_buffer_object *bo,
         struct ttm_bo_global *glob = bo->glob;
         int ret;
 
-        while (unlikely(atomic_cmpxchg(&bo->reserved, 0, 1) != 0)) {
+        while (unlikely(atomic_read(&bo->reserved) != 0)) {
                 /**
                  * Deadlock avoidance for multi-bo reserving.
                  */
@@ -249,6 +249,7 @@ int ttm_bo_reserve_locked(struct ttm_buffer_object *bo,
                         return ret;
         }
 
+        atomic_set(&bo->reserved, 1);
         if (use_sequence) {
                 /**
                  * Wake up waiters that may need to recheck for deadlock,