drm/radeon: update rptr saving logic for memory buffers

Add support for using memory buffers rather than
scratch registers.  Some rings may not be able to
write to scratch registers.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
diff --git a/drivers/gpu/drm/radeon/evergreen.c b/drivers/gpu/drm/radeon/evergreen.c
index 4b8e5c5..870009a 100644
--- a/drivers/gpu/drm/radeon/evergreen.c
+++ b/drivers/gpu/drm/radeon/evergreen.c
@@ -1364,17 +1364,25 @@
 void evergreen_ring_ib_execute(struct radeon_device *rdev, struct radeon_ib *ib)
 {
 	struct radeon_ring *ring = &rdev->ring[ib->ring];
+	u32 next_rptr;
 
 	/* set to DX10/11 mode */
 	radeon_ring_write(ring, PACKET3(PACKET3_MODE_CONTROL, 0));
 	radeon_ring_write(ring, 1);
 
 	if (ring->rptr_save_reg) {
-		uint32_t next_rptr = ring->wptr + 3 + 4;
+		next_rptr = ring->wptr + 3 + 4;
 		radeon_ring_write(ring, PACKET3(PACKET3_SET_CONFIG_REG, 1));
 		radeon_ring_write(ring, ((ring->rptr_save_reg - 
 					  PACKET3_SET_CONFIG_REG_START) >> 2));
 		radeon_ring_write(ring, next_rptr);
+	} else if (rdev->wb.enabled) {
+		next_rptr = ring->wptr + 5 + 4;
+		radeon_ring_write(ring, PACKET3(PACKET3_MEM_WRITE, 3));
+		radeon_ring_write(ring, ring->next_rptr_gpu_addr & 0xfffffffc);
+		radeon_ring_write(ring, (upper_32_bits(ring->next_rptr_gpu_addr) & 0xff) | (1 << 18));
+		radeon_ring_write(ring, next_rptr);
+		radeon_ring_write(ring, 0);
 	}
 
 	radeon_ring_write(ring, PACKET3(PACKET3_INDIRECT_BUFFER, 2));