linux-kernel - Re: [PATCH 20/21] ARM: dma-mapping: split out arch_dma_mark

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Date:   Mon, 27 Mar 2023 16:01:43 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Vineet Gupta <vgupta@...nel.org>,
        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>,
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        Michael Ellerman <mpe@...erman.id.au>,
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Subject: Re: [PATCH 20/21] ARM: dma-mapping: split out arch_dma_mark_clean()
 helper

On Mon, Mar 27, 2023 at 02:13:16PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> The arm version of the arch_sync_dma_for_cpu() function annotates pages as
> PG_dcache_clean after a DMA, but no other architecture does this here.

... because this is an arm32 specific feature. Generically, it's
PG_arch_1, which is a page flag free for architecture use. On arm32
we decided to use this to mark whether we can skip dcache writebacks
when establishing a PTE - and thus it was decided to call it
PG_dcache_clean to reflect how arm32 decided to use that bit.

This isn't just a DMA thing, there are other places that we update
the bit, such as flush_dcache_page() and copy_user_highpage().

So thinking that the arm32 PG_dcache_clean is something for DMA is
actually wrong.

Other architectures are free to do their own other optimisations
using that bit, and their implementations may be DMA-centric.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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