linux-kernel - Re: [PATCH v3 04/18] driver core: platform: Add driver dma ownership management

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Date:   Tue, 7 Dec 2021 09:30:28 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        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 v3 04/18] driver core: platform: Add driver dma ownership
 management

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