drm/msm/gpu: make ringbuffer readonly

The GPU has no business writing into the ringbuffer, let's make it
readonly to the GPU.

Fixes: 7198e6b ("drm/msm: add a3xx gpu support")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/msm_ringbuffer.c

@@ -27,7 +27,8 @@ struct msm_ringbuffer *msm_ringbuffer_new(struct msm_gpu *gpu, int id,
 	ring->id = id;
 
 	ring->start = msm_gem_kernel_new(gpu->dev, MSM_GPU_RINGBUFFER_SZ,
-		MSM_BO_WC, gpu->aspace, &ring->bo, &ring->iova);
+		MSM_BO_WC | MSM_BO_GPU_READONLY, gpu->aspace, &ring->bo,
+		&ring->iova);
 
 	if (IS_ERR(ring->start)) {
 		ret = PTR_ERR(ring->start);