2 files changed, 8 insertions, 0 deletions

diff --git a/drivers/char/drm/radeon_cp.c b/drivers/char/drm/radeon_cp.c
index 5ad43ba7b5aa..3956628b2576 100644
--- a/drivers/char/drm/radeon_cp.c
+++ b/drivers/char/drm/radeon_cp.c
@@ -1258,6 +1258,13 @@ static void radeon_test_writeback(drm_radeon_private_t * dev_priv)
                 dev_priv->writeback_works = 0;
                 DRM_INFO("writeback forced off\n");
         }
+
+        if (!dev_priv->writeback_works) {
+                /* Disable writeback to avoid unnecessary bus master transfer */
+                RADEON_WRITE(RADEON_CP_RB_CNTL, RADEON_READ(RADEON_CP_RB_CNTL) |
+                             RADEON_RB_NO_UPDATE);
+                RADEON_WRITE(RADEON_SCRATCH_UMSK, 0);
+        }
 }
 
 /* Enable or disable PCI-E GART on the chip */
diff --git a/drivers/char/drm/radeon_drv.h b/drivers/char/drm/radeon_drv.h
index e5a256f5429c..b54b8967dcd2 100644
--- a/drivers/char/drm/radeon_drv.h
+++ b/drivers/char/drm/radeon_drv.h
@@ -681,6 +681,7 @@ extern int r300_do_cp_cmdbuf(drm_device_t * dev, DRMFILE filp,
 #define RADEON_CP_RB_BASE               0x0700
 #define RADEON_CP_RB_CNTL               0x0704
 #       define RADEON_BUF_SWAP_32BIT            (2 << 16)
+#       define RADEON_RB_NO_UPDATE              (1 << 27)
 #define RADEON_CP_RB_RPTR_ADDR          0x070c
 #define RADEON_CP_RB_RPTR               0x0710
 #define RADEON_CP_RB_WPTR               0x0714