drm/i915: Use atomic for dev_priv->mm.bsd_engine_dispatch_index

Use atomic type and operands for dev_priv->mm.bsd_engine_dispatch_index
to avoid one struct_mutex locking scenario.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Zhao Yakui <yakui.zhao@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1472731101-21982-1-git-send-email-joonas.lahtinen@linux.intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -1338,7 +1338,7 @@ struct i915_gem_mm {
 	bool interruptible;
 
 	/* the indicator for dispatch video commands on two BSD rings */
-	unsigned int bsd_engine_dispatch_index;
+	atomic_t bsd_engine_dispatch_index;
 
 	/** Bit 6 swizzling required for X tiling */
 	uint32_t bit_6_swizzle_x;

drivers/gpu/drm/i915/i915_gem.c

@@ -4622,6 +4622,8 @@ i915_gem_load_init(struct drm_device *dev)
 
 	dev_priv->mm.interruptible = true;
 
+	atomic_set(&dev_priv->mm.bsd_engine_dispatch_index, 0);
+
 	spin_lock_init(&dev_priv->fb_tracking.lock);
 }
 

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1535,13 +1535,9 @@ gen8_dispatch_bsd_engine(struct drm_i915_private *dev_priv,
 	struct drm_i915_file_private *file_priv = file->driver_priv;
 
 	/* Check whether the file_priv has already selected one ring. */
-	if ((int)file_priv->bsd_engine < 0) {
-		/* If not, use the ping-pong mechanism to select one. */
-		mutex_lock(&dev_priv->drm.struct_mutex);
-		file_priv->bsd_engine = dev_priv->mm.bsd_engine_dispatch_index;
-		dev_priv->mm.bsd_engine_dispatch_index ^= 1;
-		mutex_unlock(&dev_priv->drm.struct_mutex);
-	}
+	if ((int)file_priv->bsd_engine < 0)
+		file_priv->bsd_engine = atomic_fetch_xor(1,
+			 &dev_priv->mm.bsd_engine_dispatch_index);
 
 	return file_priv->bsd_engine;
 }