drm: radeon: fix up bus mastering when writeback is disabled

When writeback isn't used, actually disable it in the hardware.

Not doing this might waste bus bandwidth or even cause memory corruption or
system crashes on systems that check bus transfers. No such incident has been
reported though.

Signed-off-by: Dave Airlie <airlied@linux.ie>

1 files changed, 7 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 */