Re: [PATCH 2/2] arch: switch the default on ARCH_HAS_SG_CHAIN

[Palmer Dabbelt]

On Mon, 19 Nov 2018 05:58:52 PST (-0800), Christoph Hellwig wrote:
> On Fri, Nov 16, 2018 at 08:52:14AM -0800, Palmer Dabbelt wrote:
> > As far as I can tell, it looks like m68k, mips, and powerpc mention an
> > IOMMU in their ports, don't set ARCH_HAS_SG_CHAIN, and with this patch set
> > won't set ARCH_NO_SG_CHAIN.
>
> m68k has no iommu, and not operations that operate on a scatterlist.
>
> mips has a trivial iommu driver (jazzdma), but I wrote the current
> instance of it, nad it is fine.
>
> powerpc has various iommu, but actually enables ARCH_HAS_SG_CHAIN
> unconditionally.
>
> > The issue is that I'm not sure how to
> > determine what constitutes a horrible legacy IOMMU, at least with respect
> > to not being able to use scatterlist chaining.
>
> It basically means someone is iterating using manual pointer arithmetics
> over a multi-element scatterlist instead of using the sg_next and
> for_each_sg helpers.

Ah, OK.  In that case, feel free to drop a

Reviewed-by: Palmer Dabbelt <palmer@xxxxxxxxxx>

in there, if you think it helps any.

Thanks for the help!