Re: [PATCH 20/21] ARM: dma-mapping: split out arch_dma_mark_clean() helper

[Russell King (Oracle)]

On Mon, Mar 27, 2023 at 02:13:16PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> 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.

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