Re: [PATCH 20/21] ARM: dma-mapping: split out arch_dma_mark_clean() helper
From: "Russell King (Oracle)" <linux@armlinux.org.uk>
Date: 2023-03-27 15:02:40
Also in:
linux-arm-kernel, linux-m68k, linux-mips, linux-riscv, linux-sh, lkml, sparclinux
On Mon, Mar 27, 2023 at 02:13:16PM +0200, Arnd Bergmann wrote:
From: Arnd Bergmann <arnd@arndb.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/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!