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Date:   Mon, 19 Nov 2018 16:48:02 -0800 (PST)
Subject:     Re: [PATCH 2/2] arch: switch the default on ARCH_HAS_SG_CHAIN
In-Reply-To: <20181119135852.GA16428@lst.de>
CC:     Christoph Hellwig <hch@lst.de>, iommu@lists.linux-foundation.org,
        linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org
From:   Palmer Dabbelt <palmer@sifive.com>
To:     Christoph Hellwig <hch@lst.de>
Message-ID: <mhng-bc691596-d40e-4dd5-b162-b767e550b92e@palmer-si-x1c4>
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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@sifive.com>

in there, if you think it helps any.

Thanks for the help!