Messages in this thread | | | From | Alexander Duyck <> | Date | Tue, 22 Aug 2023 11:58:15 -0700 | Subject | Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA |
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On Tue, Aug 22, 2023 at 11:30 AM Alexander Duyck <alexander.duyck@gmail.com> wrote: > > On Tue, Aug 22, 2023 at 8:38 AM Jakub Kicinski <kuba@kernel.org> wrote: > > > > On Tue, 22 Aug 2023 17:21:35 +0800 Yunsheng Lin wrote: > > > > .. we should also add a: > > > > > > > > WARN_ONCE(1, "misaligned DMA address, please report to netdev@"); > > > > > > As the CONFIG_PHYS_ADDR_T_64BIT seems to used widely in x86/arm/mips/powerpc, > > > I am not sure if we can really make the above assumption. > > > > > > https://elixir.free-electrons.com/linux/v6.4-rc6/K/ident/CONFIG_PHYS_ADDR_T_64BIT > > > > Huh, it's actually used a lot less than I anticipated! > > > > None of the x86/arm/mips/powerpc systems matter IMHO - the only _real_ > > risk is something we don't know about returning non-aligned addresses. > > > > Unless we know about specific problems I'd suggest we took the simpler > > path rather than complicating the design for systems which may not > > exist. > > > > Alex, do you know of any such cases? Some crazy swiotlb setting? > > WDYT about this in general? > > There may be scenarios where if bounce buffers are used the page may > not be aligned. It all comes down to how > swiotlb_tbl_map_single(https://elixir.free-electrons.com/linux/v6.5-rc7/C/ident/swiotlb_tbl_map_single) > is called. In the IOMMU case it looks like they take the extra step of > passing an alignment value, but it looks like for the other two cases > they don't. > > Changing that behavior wouldn't take much though. Basically we would > just need to do something like look at the size and address and if > they are both page aligned then we could specify a page alignment for > the DMA mapping.
Actually I take that back. It looks like in the bounce case there is already code that will look for PAGE_SIZE aligned blocks if the request is for PAGE_SIZE or larger. So there shouldn't be any cases where a PAGE_SIZE request is not PAGE_SIZE aligned in DMA that I am aware of.
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