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

On 27 Mar 2023, at 13:13, Arnd Bergmann <[email protected]> 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.

FYI [1] proposed this same change a while ago but its justification was
flawed (which was my objection at the time, not the diff itself).

Jess

[1] https://lore.kernel.org/all/[email protected]

> 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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