linux-kernel - Re: fix DMA ops layering violations in vmwgfx

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Date:   Tue, 8 Jan 2019 09:51:45 +0000
From:   Thomas Hellstrom <thellstrom@...are.com>
To:     "hch@....de" <hch@....de>
CC:     "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Linux-graphics-maintainer <Linux-graphics-maintainer@...are.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: fix DMA ops layering violations in vmwgfx

Hi, Christoph,

On Sat, 2019-01-05 at 09:01 +0100, Christoph Hellwig wrote:
> Hi Thomas,
> 
> vmwgfx has been doing some odd checks based on DMA ops which rely
> on deep DMA mapping layer internals, and I think the changes in
> Linux 4.21 finally broke most of these implicit assumptions.

Thanks. 
What we're really trying to do here is to try to detect the situation
where DMA remapping using hardware IOMMUs is going on but memory is
still coherent, since the driver can currently only work with coherent
memory[1]. Currently we use intel_iommu_enabled to detect this
situation, but it would be really helpful if there were a generic bool
that advertizes this situation since we need to deal with other IOMMUs
as well going forward. Any suggestion?

Comments on the patches separately.

Thanks,
Thomas

> 
> The real fix is in patch 3, but I think the others are important
> to make it clear what is actually going on.