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

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Message-Id: <34f3e043-20eb-40b2-a7f5-b1b98a0d452a@app.fastmail.com>
Date:   Thu, 30 Mar 2023 09:10:57 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Conor Dooley" <conor@...nel.org>,
        "Arnd Bergmann" <arnd@...nel.org>
Cc:     linux-kernel@...r.kernel.org, "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>, guoren <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>,
        "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>,
        "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-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-m68k@...ts.linux-m68k.org,
        linux-mips@...r.kernel.org,
        "linux-openrisc@...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,
        "Samuel Holland" <samuel@...lland.org>
Subject: Re: [PATCH 08/21] riscv: dma-mapping: only invalidate after DMA, not flush

On Wed, Mar 29, 2023, at 22:48, Conor Dooley wrote:
> On Mon, Mar 27, 2023 at 02:13:04PM +0200, Arnd Bergmann 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.
>
> So does this need a
> Fixes: 1631ba1259d6 ("riscv: Add support for non-coherent devices using 
> zicbom extension")
> then, even if the cacheline really should not have been touched by the
> CPU?
> Also, minor typo, s/drive/driver/.

done

> In the thread we had that sparked this, I went digging for the source of
> the flushes, and it came from a review comment:
> https://lore.kernel.org/linux-riscv/342e3c12-ebb0-badf-7d4c-c444a2b842b2@sholland.org/

Ah, so the comment that led to it was 

"For arch_sync_dma_for_cpu(DMA_BIDIRECTIONAL), we expect the CPU to have
written to the buffer, so this should flush, not invalidate."

which sounds like Samuel just misunderstood what "bidirectional"
means: the comment implies that both the cpu and the device access
the buffer before arch_sync_dma_for_cpu(DMA_BIDIRECTIONAL), but
this is not allowed. Instead, the point is that the device may both
read and write the buffer, requiring that we must do a writeback
at arch_sync_dma_for_device(DMA_BIDIRECTIONAL) and an invalidate
at arch_sync_dma_for_cpu(DMA_BIDIRECTIONAL).

The comment about arch_sync_dma_for_device(DMA_FROM_DEVICE) (in the
same email) seems equally confused. It's of course easy to
misunderstand these, and many others have gotten confused in
similar ways before.

> But *surely* if no other arch needs to do that, then we are safe to also
> not do it... Your logic seems right by me at least, especially given the
> lack of flushes elsewhere.

Right, I remove the extra writeback from powerpc, parisc and microblaze
for the same reason. Those appear to only be there because they used the
same function for _for_device() as for _for_cpu(), not because someone
thought they were required.

> Reviewed-by: Conor Dooley <conor.dooley@...rochip.com>

Thanks!

     Arnd