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

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Date:   Wed, 19 Apr 2023 07:22:50 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     arnd@...nel.org
CC:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        vgupta@...nel.org, linux@...linux.org.uk,
        neil.armstrong@...aro.org, linus.walleij@...aro.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, guoren@...nel.org,
        bcain@...cinc.com, geert@...ux-m68k.org, monstr@...str.eu,
        tsbogend@...ha.franken.de, dinguyen@...nel.org, shorne@...il.com,
        deller@....de, mpe@...erman.id.au, christophe.leroy@...roup.eu,
        Paul Walmsley <paul.walmsley@...ive.com>, dalias@...c.org,
        glaubitz@...sik.fu-berlin.de, davem@...emloft.net,
        jcmvbkbc@...il.com, Christoph Hellwig <hch@....de>,
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        Conor Dooley <conor.dooley@...rochip.com>,
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        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-openrisc@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org
Subject:     Re: [PATCH 08/21] riscv: dma-mapping: only invalidate after DMA, not flush

On Mon, 27 Mar 2023 05:13:04 PDT (-0700), arnd@...nel.org wrote:
> From: Arnd Bergmann <arnd@...db.de>
>
> 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.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  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;

Acked-by: Palmer Dabbelt <palmer@...osinc.com>