Re: [PATCH v3 2/3] riscv: dma-mapping: skip invalidation before bidirectional DMA

[Jisheng Zhang]

On Thu, Aug 17, 2023 at 12:23:35AM +0100, Prabhakar wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> For a DMA_BIDIRECTIONAL transfer, the caches have to be cleaned
> first to let the device see data written by the CPU, and invalidated
> after the transfer to let the CPU see data written by the device.
> 
> riscv also invalidates the caches before the transfer, which does
> not appear to serve any purpose.
> 
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> Reviewed-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
> Acked-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> Acked-by: Guo Ren <guoren@xxxxxxxxxx>
> Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
> ---
> v2->v3
> * No change
> 
> v1->v2
>  * Included RB and ACKs
> ---
>  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 94614cf61cdd..fc6377a64c8d 100644
> --- a/arch/riscv/mm/dma-noncoherent.c
> +++ b/arch/riscv/mm/dma-noncoherent.c
> @@ -25,7 +25,7 @@ void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
>  		ALT_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
>  		break;
>  	case DMA_BIDIRECTIONAL:
> -		ALT_CMO_OP(flush, vaddr, size, riscv_cbom_block_size);
> +		ALT_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);

The code could be simplified a lot since after this patch, the action
is always "clean".

Thanks
>  		break;
>  	default:
>  		break;
> -- 
> 2.34.1
>