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Subject: Re: [PATCH 08/21] riscv: dma-mapping: only invalidate after DMA, not
 flush
From:   Jessica Clarke <jrtc27@jrtc27.com>
In-Reply-To: <20230327121317.4081816-9-arnd@kernel.org>
Date:   Wed, 29 Mar 2023 22:51:54 +0100
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Vineet Gupta <vgupta@kernel.org>,
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References: <20230327121317.4081816-1-arnd@kernel.org>
 <20230327121317.4081816-9-arnd@kernel.org>
To:     Arnd Bergmann <arnd@kernel.org>
Precedence: bulk

On 27 Mar 2023, at 13:13, Arnd Bergmann <arnd@kernel.org> wrote:
>=20
> From: Arnd Bergmann <arnd@arndb.de>
>=20
> 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/20220818165105.99746-1-s.miroshnichenko@yadro.=
com

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> arch/riscv/mm/dma-noncoherent.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>=20
> 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;
> --=20
> 2.39.2
>=20
>=20
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