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From:   Tom Murphy <murphyt7@tcd.ie>
Date:   Mon, 7 Sep 2020 21:18:50 +0100
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Subject: Re: [PATCH V2 5/5] DO NOT MERGE: iommu: disable list appending in dma-iommu
To:     Christoph Hellwig <hch@infradead.org>
Cc:     iommu@lists.linux-foundation.org,
        David Woodhouse <dwmw2@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        intel-gfx@lists.freedesktop.org
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On Mon, 7 Sep 2020 at 08:00, Christoph Hellwig <hch@infradead.org> wrote:
>
> On Thu, Sep 03, 2020 at 09:18:37PM +0100, Tom Murphy wrote:
> > Disable combining sg segments in the dma-iommu api.
> > Combining the sg segments exposes a bug in the intel i915 driver which
> > causes visual artifacts and the screen to freeze. This is most likely
> > because of how the i915 handles the returned list. It probably doesn't
> > respect the returned value specifying the number of elements in the list
> > and instead depends on the previous behaviour of the intel iommu driver
> > which would return the same number of elements in the output list as in
> > the input list.
>
> So what is the state of addressing this properly in i915?  IF we can't

I think this is the latest on addressing this issue:
https://patchwork.kernel.org/cover/11306999/

tl;dr: some people seem to be looking at it but I'm not sure if it's
being actively worked on

> get it done ASAP I wonder if we need a runtime quirk to disable
> merging instead of blocking this conversion..

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.