linux-kernel - Re: [PATCH v1 5/8] iommu/amd: Use iommu_attach/detach

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Message-ID: <YgplyyjofwlM+1tc@8bytes.org>
Date:   Mon, 14 Feb 2022 15:23:07 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Will Deacon <will@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>, rafael@...nel.org,
        Diana Craciun <diana.craciun@....nxp.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Stuart Yoder <stuyoder@...il.com>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Li Yang <leoyang.li@....com>,
        Dmitry Osipenko <digetx@...il.com>,
        iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 5/8] iommu/amd: Use iommu_attach/detach_device()

On Mon, Feb 14, 2022 at 10:02:36AM -0400, Jason Gunthorpe wrote:
> That works for VFIO, but it doesn't work for other in-kernel
> drivers.. Is there something ensuring the group is only the GPU and
> sound device? Is the GPU never an addin card?

GPUs supporting this functionality are always iGPUs, AFAIK.

> I'd say the right way to code this after Lu's series to have both the
> GPU and sound driver call iommu_attach_device() during their probe()'s
> and specify the identity domain as the attaching domain.
> 
> That would be quite similar to how the Tegra drivers got arranged.
> 
> And then maybe someone could better guess what the "sound driver" is
> since it would be marked with an iommu_attach_device() call.

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.

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.

Regards,

	Joerg