Re: [PATCH v3 2/2] gpu: nova-core: fix barrier usage in GSP->CPU messaging path

[Eliot Courtney]

On Thu Aug 20, 2026 at 2:28 AM JST, Gary Guo wrote:
> In the GSP->CPU messaging path, the code reads the write pointer from GSP,
> reads the response and advances the read pointer.
>
> A LOAD->LOAD ordering is required after the write pointer read and the data
> read. Add it as this is currently missing.
>
> A LOAD->STORE ordering is required after the data read and the advance of
> read pointer. Currently a Rust `SeqCst` barrier is used, which roughly maps
> to `smp_mb(Full)`; this however does not order DMA operations (notably on
> ARM, the generate barrier orders inner shareable and not outer shareable,
> which is ordered by `dma_mb`). This ordering does not need to be in between
> read pointer read and write, because it's for ordering between the ring
> buffer data and the pointer; the RMW operation does not internally need a
> barrier (nor it has to be atomic, as CPU pointers are updated by CPU only),
> so move it to before the RMW sequence for clarity.
>
> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
> ---

Reviewed-by: Eliot Courtney <ecourtney@xxxxxxxxxx>