drm/i915: Remove locking for get_tiling

Since we are not concerned with userspace racing itself with set-tiling
(the order is indeterminant even if we take a lock), then we can safely
read back the single obj->tiling_mode and do the static lookup of
swizzle mode without having to take a lock.

get-tiling is reasonably frequent due to the back-channel passing around
of tiling parameters in DRI2/DRI3.

v2: Make tiling_mode a full unsigned int so that we can trivially use it
with READ_ONCE(). Separating it out into manual control over the flags
field was too noisy for a simple patch. Note that we could use the lower
bits of obj->stride for the tiling mode.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470388464-28458-16-git-send-email-chris@chris-wilson.co.uk

2 files changed, 11 insertions, 14 deletions

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 31a614fe9ed7..f18d8761305c 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2183,10 +2183,6 @@ struct drm_i915_gem_object {
         unsigned int madv:2;
 
         /**
-         * Current tiling mode for the object.
-         */
-        unsigned int tiling_mode:2;
-        /**
          * Whether the tiling parameters for the currently associated fence
          * register have changed. Note that for the purposes of tracking
          * tiling changes we also treat the unfenced register, the register
@@ -2218,6 +2214,14 @@ struct drm_i915_gem_object {
 
         atomic_t frontbuffer_bits;
 
+        /**
+         * Current tiling mode for the object.
+         */
+        unsigned int tiling_mode;
+
+        /** Current tiling stride for the object, if it's tiled. */
+        uint32_t stride;
+
         unsigned int has_wc_mmap;
         /** Count of VMA actually bound by this object */
         unsigned int bind_count;
@@ -2245,9 +2249,6 @@ struct drm_i915_gem_object {
         struct i915_gem_active last_write;
         struct i915_gem_active last_fence;
 
-        /** Current tiling stride for the object, if it's tiled. */
-        uint32_t stride;
-
         /** References from framebuffers, locks out tiling changes. */
         unsigned long framebuffer_references;
 
diff --git a/drivers/gpu/drm/i915/i915_gem_tiling.c b/drivers/gpu/drm/i915/i915_gem_tiling.c
index b7f9875f69b4..c0e01333bddf 100644
--- a/drivers/gpu/drm/i915/i915_gem_tiling.c
+++ b/drivers/gpu/drm/i915/i915_gem_tiling.c
@@ -303,10 +303,8 @@ i915_gem_get_tiling(struct drm_device *dev, void *data,
         if (!obj)
                 return -ENOENT;
 
-        mutex_lock(&dev->struct_mutex);
-
-        args->tiling_mode = obj->tiling_mode;
-        switch (obj->tiling_mode) {
+        args->tiling_mode = READ_ONCE(obj->tiling_mode);
+        switch (args->tiling_mode) {
         case I915_TILING_X:
                 args->swizzle_mode = dev_priv->mm.bit_6_swizzle_x;
                 break;
@@ -330,8 +328,6 @@ i915_gem_get_tiling(struct drm_device *dev, void *data,
         if (args->swizzle_mode == I915_BIT_6_SWIZZLE_9_10_17)
                 args->swizzle_mode = I915_BIT_6_SWIZZLE_9_10;
 
-        i915_gem_object_put(obj);
-        mutex_unlock(&dev->struct_mutex);
-
+        i915_gem_object_put_unlocked(obj);
         return 0;
 }