Date:   Tue, 7 Dec 2021 09:30:28 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Lu Baolu <baolu.lu@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Joerg Roedel <joro@8bytes.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Kevin Tian <kevin.tian@intel.com>,
        Ashok Raj <ashok.raj@intel.com>, Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        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>,
        Thierry Reding <thierry.reding@gmail.com>,
        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 v3 04/18] driver core: platform: Add driver dma ownership
 management
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On Tue, Dec 07, 2021 at 05:25:04AM -0800, Christoph Hellwig wrote:
> On Tue, Dec 07, 2021 at 09:16:27AM -0400, Jason Gunthorpe wrote:
> > Yes, the suggestion was to put everything that 'if' inside a function
> > and then of course a matching undo function.
> 
> Can't we simplify things even more?  Do away with the DMA API owner
> entirely, and instead in iommu_group_set_dma_owner iterate over all
> devices in a group and check that they all have the no_dma_api flag
> set (plus a similar check on group join).  With that most of the
> boilerplate code goes away entirely in favor of a little more work at
> iommu_group_set_dma_owner time.

Robin suggested something like this already.

The locking doesn't work out, we can't nest device_lock()'s safely
without ABBA deadlocks, and can't touch the dev->driver without the
device_lock.

Jason