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Date:   Mon, 14 Feb 2022 11:00:59 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Joerg Roedel <joro@8bytes.org>
Cc:     Lu Baolu <baolu.lu@linux.intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Christoph Hellwig <hch@infradead.org>,
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        Ashok Raj <ashok.raj@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Will Deacon <will@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>, rafael@kernel.org,
        Diana Craciun <diana.craciun@oss.nxp.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Eric Auger <eric.auger@redhat.com>,
        Liu Yi L <yi.l.liu@intel.com>,
        Jacob jun Pan <jacob.jun.pan@intel.com>,
        Chaitanya Kulkarni <kch@nvidia.com>,
        Stuart Yoder <stuyoder@gmail.com>,
        Laurentiu Tudor <laurentiu.tudor@nxp.com>,
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        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Li Yang <leoyang.li@nxp.com>,
        Dmitry Osipenko <digetx@gmail.com>,
        iommu@lists.linux-foundation.org, linux-pci@vger.kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 5/8] iommu/amd: Use iommu_attach/detach_device()
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On Mon, Feb 14, 2022 at 03:23:07PM +0100, Joerg Roedel wrote:

> Device drivers calling into iommu_attach_device() is seldom a good
> idea.  In this case the sound device has some generic hardware
> interface so that an existing sound driver can be re-used. Making this
> driver call iommu-specific functions for some devices is something hard
> to justify.

Er, so this is transparent to the generic sound device? I guess
something fixed up the dma_api on that device to keep working?

But, then, the requirement is that nobody is using the dma API when we
make this change?

> With sub-groups on the other hand it would be a no-brainer, because the
> sound device would be in a separate sub-group. Basically any device in
> the same group as the GPU would be in a separate sub-group.

I don't think it matters how big/small the group is, only that when we
change the domain we know everything flowing through the domain is
still happy.

Jason