drm/v3d: Drop the wait for L2T flush to complete.

According to Dave, once you've started an L2T flush, all L2T accesses
will be blocked until the flush completes.  This fixes a consistent
3-4ms stall between the ioctl and running the job, and 3DMMES Taiji
goes from 27fps to 110fps.

v2: Leave a note about why we don't need to wait for completion.
Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 57692c94 ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
Reviewed-by: Dave Emett <david.emett@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203222438.25417-4-eric@anholt.net

drivers/gpu/drm/v3d/v3d_gem.c

@@ -143,13 +143,13 @@ v3d_invalidate_l2(struct v3d_dev *v3d, int core)
 static void
 v3d_flush_l2t(struct v3d_dev *v3d, int core)
 {
+	/* While there is a busy bit (V3D_L2TCACTL_L2TFLS), we don't
+	 * need to wait for completion before dispatching the job --
+	 * L2T accesses will be stalled until the flush has completed.
+	 */
 	V3D_CORE_WRITE(core, V3D_CTL_L2TCACTL,
 		       V3D_L2TCACTL_L2TFLS |
 		       V3D_SET_FIELD(V3D_L2TCACTL_FLM_FLUSH, V3D_L2TCACTL_FLM));
-	if (wait_for(!(V3D_CORE_READ(core, V3D_CTL_L2TCACTL) &
-		       V3D_L2TCACTL_L2TFLS), 100)) {
-		DRM_ERROR("Timeout waiting for L2T flush\n");
-	}
 }
 
 /* Invalidates the slice caches.  These are read-only caches. */