linux-kernel - Re: [PATCH 2/2] arch: switch the default on ARCH_HAS_SG

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Date:   Mon, 19 Nov 2018 14:58:52 +0100
From:   Christoph Hellwig <hch@....de>
To:     Palmer Dabbelt <palmer@...ive.com>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] arch: switch the default on ARCH_HAS_SG_CHAIN

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.