On Sat, Sep 18, 2021 at 11:37:22AM +0300, Mike Rapoport wrote: > On Sat, Sep 18, 2021 at 07:18:43AM +0200, Christoph Hellwig wrote: > > On Sat, Sep 18, 2021 at 12:22:47AM +0300, Mike Rapoport wrote: > > > I did some digging and it seems that the most "generic" way to check if a > > > page is in RAM is page_is_ram(). It's not 100% bullet proof as it'll give > > > false negatives for architectures that do not register "System RAM", but > > > those are not using dma_map_resource() anyway and, apparently, never would. > > > > The downside of page_is_ram is that it looks really expensiv for > > something done at dma mapping time. > > Indeed :( > But pfn_valid is plain wrong... > I'll keep digging. I did some more archaeology and it that check for pfn_valid() was requested by arm folks because their MMU may have troubles with alias mappings with different attributes and so they made the check to use a false assumption that pfn_valid() == "RAM". As this WARN_ON(pfn_valid()) is only present in dma_map_resource() it's probably safe to drop it entirely. Otherwise the simplest way would be to hide it behind something like ARCH_WANTS_DMA_NOT_RAM and make arm/arm64 select it. Thoughts? -- Sincerely yours, Mike.
