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Date:   Tue, 8 Sep 2020 06:55:10 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     Tom Murphy <murphyt7@tcd.ie>
Cc:     intel-gfx@lists.freedesktop.org,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Christoph Hellwig <hch@infradead.org>,
        iommu@lists.linux-foundation.org,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        David Woodhouse <dwmw2@infradead.org>
Subject: Re: [PATCH V2 5/5] DO NOT MERGE: iommu: disable list appending in
 dma-iommu
Message-ID: <20200908055510.GA19078@infradead.org>
References: <20200903201839.7327-1-murphyt7@tcd.ie>
 <20200903201839.7327-6-murphyt7@tcd.ie>
 <20200907070035.GA25114@infradead.org>
 <CALQxJute8_y=JsW4UV1awSccOjxT_1OyPdymq=R_PurVQzENeQ@mail.gmail.com>
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On Tue, Sep 08, 2020 at 06:36:19AM +0100, Christoph Hellwig wrote:
> On Mon, Sep 07, 2020 at 09:18:50PM +0100, Tom Murphy wrote:
> > Yeah we talked about passing an attr to map_sg to disable merging at
> > the following microconfernce:
> > https://linuxplumbersconf.org/event/7/contributions/846/
> > As far as I can remember everyone seemed happy with that solution. I
> > won't be working on this though as I don't have any more time to
> > dedicate to this. It seems Lu Baolu will take over this.
> 
> I'm absolutely again passing a flag.  Tha just invites further
> abuse.  We need a PCI ID based quirk or something else that can't
> be as easily abused.

Also, I looked at i915 and there are just three dma_map_sg callers.
The two dmabuf related ones are fixed by Marek in his series, leaving
just the one in i915_gem_gtt_prepare_pages, which does indeed look
very fishy.  But if that one is so hard to fix it can just be replaced
by an open coded for_each_sg loop that contains manual dma_map_page
calls.