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>

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 */

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