linux-kernel - Re: [PATCH 09/21] riscv: dma-mapping: skip invalidation before bidirectional DMA

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Date:   Fri, 5 May 2023 13:47:03 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Arnd Bergmann <arnd@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, Vineet Gupta <vgupta@...nel.org>,
        Will Deacon <will@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Brian Cain <bcain@...cinc.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Michal Simek <monstr@...str.eu>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Stafford Horne <shorne@...il.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Rich Felker <dalias@...c.org>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        "David S. Miller" <davem@...emloft.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
        Conor Dooley <conor.dooley@...rochip.com>,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-oxnas@...ups.io,
        linux-csky@...r.kernel.org, linux-hexagon@...r.kernel.org,
        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 09/21] riscv: dma-mapping: skip invalidation before
 bidirectional DMA

On Mon, Mar 27, 2023 at 8:15 PM Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
>
> 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.
Yes, we can't guarantee the CPU pre-load cache lines randomly during
dma working.

But I've two purposes to keep invalidates before dma transfer:
 - We clearly tell the CPU these cache lines are invalid. The caching
algorithm would use these invalid slots first instead of replacing
valid ones.
 - Invalidating is very cheap. Actually, flush and clean have the same
performance in our machine.

So, how about:

diff --git a/arch/riscv/mm/dma-noncoherent.c b/arch/riscv/mm/dma-noncoherent.c
index d919efab6eba..2c52fbc15064 100644
--- a/arch/riscv/mm/dma-noncoherent.c
+++ b/arch/riscv/mm/dma-noncoherent.c
@@ -22,8 +22,6 @@ 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_FROM_DEVICE:
-               ALT_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
-               break;
        case DMA_BIDIRECTIONAL:
                ALT_CMO_OP(flush, vaddr, size, riscv_cbom_block_size);
                break;
@@ -42,7 +40,7 @@ void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size,
                break;
        case DMA_FROM_DEVICE:
        case DMA_BIDIRECTIONAL:
                /* I'm not sure all drivers have guaranteed cacheline
alignment. If not, this inval would cause problems */
-               ALT_CMO_OP(flush, vaddr, size, riscv_cbom_block_size);
+               ALT_CMO_OP(inval, vaddr, size, riscv_cbom_block_size);
                break;
        default:
                break;

>
> 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 640f4c496d26..69c80b2155a1 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);
>                 break;
>         default:
>                 break;
> --
> 2.39.2
>


-- 
Best Regards
 Guo Ren