Comment # 48 on bug 78453 from Michel Dänzer

(In reply to comment #42)
> There is only one buffer_map(READ), in si_get_backend_mask, right after the
> buffer_map(WRITE) from si_get_backend_mask.

Please make sure the kernel has the commit(s) necessary to provide the backend
mask to userspace, so si_get_backend_mask() doesn't need to fall back to this
method.