linux-kernel - Re: [PATCH V2 5/5] DO NOT MERGE: iommu: disable list appending in dma-iommu

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Message-ID: <20200907070035.GA25114@infradead.org>
Date:   Mon, 7 Sep 2020 08:00:35 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Tom Murphy <murphyt7@....ie>
Cc:     iommu@...ts.linux-foundation.org,
        David Woodhouse <dwmw2@...radead.org>,
        linux-kernel@...r.kernel.org,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        intel-gfx@...ts.freedesktop.org
Subject: Re: [PATCH V2 5/5] DO NOT MERGE: iommu: disable list appending in
 dma-iommu

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