drm/i915: Fix VCS ring selection after uapi decoupling

This got broken in:

   commit de1add36
   Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
   Date:   Fri Jan 15 15:12:50 2016 +0000

       drm/i915: Decouple execbuf uAPI from internal implementation

BSD ring flags need to be shifted before they can be considered
indices into the ring array.

Reported by Zhipeng Gong.

v2: Simplify the code. (Chris Wilson)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhipeng Gong <zhipeng.gong@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1453902069-31353-1-git-send-email-tvrtko.ursulin@linux.intel.com
Testcase: igt/gem_exec_basic # bdw-gt3

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1401,6 +1401,7 @@ eb_select_ring(struct drm_i915_private *dev_priv,
 			bsd_idx = gen8_dispatch_bsd_ring(dev_priv, file);
 		} else if (bsd_idx >= I915_EXEC_BSD_RING1 &&
 			   bsd_idx <= I915_EXEC_BSD_RING2) {
+			bsd_idx >>= I915_EXEC_BSD_SHIFT;
 			bsd_idx--;
 		} else {
 			DRM_DEBUG("execbuf with unknown bsd ring: %u\n",

include/uapi/drm/i915_drm.h

@@ -772,10 +772,12 @@ struct drm_i915_gem_execbuffer2 {
 #define I915_EXEC_HANDLE_LUT		(1<<12)
 
 /** Used for switching BSD rings on the platforms with two BSD rings */
-#define I915_EXEC_BSD_MASK		(3<<13)
-#define I915_EXEC_BSD_DEFAULT		(0<<13) /* default ping-pong mode */
-#define I915_EXEC_BSD_RING1		(1<<13)
-#define I915_EXEC_BSD_RING2		(2<<13)
+#define I915_EXEC_BSD_SHIFT	 (13)
+#define I915_EXEC_BSD_MASK	 (3 << I915_EXEC_BSD_SHIFT)
+/* default ping-pong mode */
+#define I915_EXEC_BSD_DEFAULT	 (0 << I915_EXEC_BSD_SHIFT)
+#define I915_EXEC_BSD_RING1	 (1 << I915_EXEC_BSD_SHIFT)
+#define I915_EXEC_BSD_RING2	 (2 << I915_EXEC_BSD_SHIFT)
 
 /** Tell the kernel that the batchbuffer is processed by
  *  the resource streamer.