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Message-ID: <52eef604-085e-acf9-f3ee-41ba9d8441d2@csgroup.eu>
Date:   Mon, 27 Mar 2023 12:56:33 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Arnd Bergmann <arnd@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Arnd Bergmann <arnd@...db.de>, Vineet Gupta <vgupta@...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>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        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>,
        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>,
        Christoph Hellwig <hch@....de>,
        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-snps-arc@...ts.infradead.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-oxnas@...ups.io" <linux-oxnas@...ups.io>,
        "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
        "linux-hexagon@...r.kernel.org" <linux-hexagon@...r.kernel.org>,
        "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-openrisc@...r.kernel.org" <linux-openrisc@...r.kernel.org>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-xtensa@...ux-xtensa.org" <linux-xtensa@...ux-xtensa.org>
Subject: Re: [PATCH 06/21] powerpc: dma-mapping: minimize for_cpu flushing



Le 27/03/2023 à 14:13, Arnd Bergmann a écrit :
> From: Arnd Bergmann <arnd@...db.de>
> 
> The powerpc dma_sync_*_for_cpu() variants do more flushes than on other
> architectures. Reduce it to what everyone else does:
> 
>   - No flush is needed after data has been sent to a device
> 
>   - When data has been received from a device, the cache only needs to
>     be invalidated to clear out cache lines that were speculatively
>     prefetched.
> 
> In particular, the second flushing of partial cache lines of bidirectional
> buffers is actively harmful -- if a single cache line is written by both
> the CPU and the device, flushing it again does not maintain coherency
> but instead overwrite the data that was just received from the device.

Hum ..... Who is right ?

That behaviour was introduced by commit 03d70617b8a7 ("powerpc: Prevent 
memory corruption due to cache invalidation of unaligned DMA buffer")

I think your commit log should explain why that commit was wrong, and 
maybe say that your patch is a revert of that commit ?

Christophe


> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>   arch/powerpc/mm/dma-noncoherent.c | 18 ++++--------------
>   1 file changed, 4 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/powerpc/mm/dma-noncoherent.c b/arch/powerpc/mm/dma-noncoherent.c
> index f10869d27de5..e108cacf877f 100644
> --- a/arch/powerpc/mm/dma-noncoherent.c
> +++ b/arch/powerpc/mm/dma-noncoherent.c
> @@ -132,21 +132,11 @@ void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size,
>   	switch (direction) {
>   	case DMA_NONE:
>   		BUG();
> -	case DMA_FROM_DEVICE:
> -		/*
> -		 * invalidate only when cache-line aligned otherwise there is
> -		 * the potential for discarding uncommitted data from the cache
> -		 */
> -		if ((start | end) & (L1_CACHE_BYTES - 1))
> -			__dma_phys_op(start, end, DMA_CACHE_FLUSH);
> -		else
> -			__dma_phys_op(start, end, DMA_CACHE_INVAL);
> -		break;
> -	case DMA_TO_DEVICE:		/* writeback only */
> -		__dma_phys_op(start, end, DMA_CACHE_CLEAN);
> +	case DMA_TO_DEVICE:
>   		break;
> -	case DMA_BIDIRECTIONAL:	/* writeback and invalidate */
> -		__dma_phys_op(start, end, DMA_CACHE_FLUSH);
> +	case DMA_FROM_DEVICE:
> +	case DMA_BIDIRECTIONAL:
> +		__dma_phys_op(start, end, DMA_CACHE_INVAL);
>   		break;
>   	}
>   }

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