Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

[Alexander Duyck]

On Tue, Aug 22, 2023 at 11:30 AM Alexander Duyck
<alexander.duyck@xxxxxxxxx> wrote:
>
> On Tue, Aug 22, 2023 at 8:38 AM Jakub Kicinski <kuba@xxxxxxxxxx> 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.