Re: [PATCH 08/21] riscv: dma-mapping: only invalidate after DMA, not flush

On Mon, Mar 27, 2023 at 02:13:04PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <[email protected]>
> 
> No other architecture intentionally writes back dirty cache lines into
> a buffer that a device has just finished writing into. If the cache is
> clean, this has no effect at all, but

> if a cacheline in the buffer has
> actually been written by the CPU,  there is a drive bug that is likely
> made worse by overwriting that buffer.

So does this need a
Fixes: 1631ba1259d6 ("riscv: Add support for non-coherent devices using zicbom extension")
then, even if the cacheline really should not have been touched by the
CPU?
Also, minor typo, s/drive/driver/.

In the thread we had that sparked this, I went digging for the source of
the flushes, and it came from a review comment:
https://lore.kernel.org/linux-riscv/[email protected]/
But *surely* if no other arch needs to do that, then we are safe to also
not do it... Your logic seems right by me at least, especially given the
lack of flushes elsewhere.
Reviewed-by: Conor Dooley <[email protected]>

Cheers,
Conor.

> Signed-off-by: Arnd Bergmann <[email protected]>
> ---
>  arch/riscv/mm/dma-noncoherent.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/mm/dma-noncoherent.c b/arch/riscv/mm/dma-noncoherent.c
> index d919efab6eba..640f4c496d26 100644
> --- a/arch/riscv/mm/dma-noncoherent.c
> +++ b/arch/riscv/mm/dma-noncoherent.c
> @@ -42,7 +42,7 @@ void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size,
>  		break;
>  	case DMA_FROM_DEVICE:
>  	case DMA_BIDIRECTIONAL:
> -		ALT_CMO_OP(flush, vaddr, size, riscv_cbom_block_size);
> +		ALT_CMO_OP(inval, vaddr, size, riscv_cbom_block_size);
>  		break;
>  	default:
>  		break;
> -- 
> 2.39.2
>

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