On Fri, Mar 31, 2023 at 04:06:37PM +0200, Arnd Bergmann wrote: > On Mon, Mar 27, 2023, at 17:01, Russell King (Oracle) wrote: > > 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. > > The flag is used the same way on most architectures, though some > use the opposite polarity and call it PG_dcache_dirty. The only > other architecture that uses it for DMA is ia64, with the difference > being that this also marks the page as clean even for coherent > DMA, not just when doing a flush as part of noncoherent DMA. > > Based on Robin's reply it sounds that this is not a valid assumption > on Arm, if a coherent DMA can target a dirty dcache line without > cleaning it. The other thing to note here is that PG_dcache_clean doesn't have much meaning on modern CPUs with PIPT caches. For these, cache_is_vipt_nonaliasing() will be true, and cache_ops_need_broadcast() will be false. Firstly, if we're using coherent DMA, then PG_dcache_clean is intentionally not touched, because the data cache isn't cleaned in any way by DMA operations. flush_dcache_page() turns into a no-op apart from clearing PG_dcache_clean if it was set. __sync_icache_dcache() will do nothing for non-executable pages, but will write-back a page that isn't marked PG_dcache_clean to ensure that it is visible to the instruction stream. This is only used to ensure that a the instructions are visible to a newly established executable mapping when e.g. the page has been DMA'd in. The default state of PG_dcache_clean is zero on any new allocation, so this has the effect of causing any executable page to be flushed such that the instruction stream can see the instructions, but only for the first establishment of the mapping. That means that e.g. libc text pages don't keep getting flushed on the start of every program. update_mmu_cache() isn't compiled, so it's use of PG_dcache_clean is irrelevant. v6_copy_user_highpage_aliasing() won't be called because we're not using an aliasing cache. So, for modern ARM systems with DMA-coherent PG_dcache_clean only serves for the __sync_icache_dcache() optimisation. ARMs use of this remains valid in this circumstance. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!
