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

drivers/gpu/drm/i915/i915_drv.h

@@ -2182,10 +2182,6 @@ 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
@@ -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;
 

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;
 }