linux-kernel - Re: [PATCH v3 01/18] iommu: Add device dma ownership set/release interfaces

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Message-ID: <Ya4eXZtdMGhEP7GO@infradead.org>
Date:   Mon, 6 Dec 2021 06:29:49 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        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 01/18] iommu: Add device dma ownership set/release
 interfaces

On Mon, Dec 06, 2021 at 02:35:55PM +0100, Joerg Roedel wrote:
> >                 enum iommu_dma_owner type, void *owner_cookie);
> >         void iommu_device_release_dma_owner(struct device *dev,
> >                 enum iommu_dma_owner type);
> 
> It the owner is a group-wide setting, it should be called with the group
> instead of the device. I have seen the group-specific funcitons are
> added later, but that leaves the question why the device-specific ones
> are needed at all.

They aren't really.  A lot of bus drivers need helpers to set/release
the dma API domain if there is an iommu group, but tegra which actually
sets a non-default value would be much better off with just open coding
them.

> > @@ -621,6 +624,7 @@ struct iommu_group *iommu_group_alloc(void)
> >  	INIT_LIST_HEAD(&group->devices);
> >  	INIT_LIST_HEAD(&group->entry);
> >  	BLOCKING_INIT_NOTIFIER_HEAD(&group->notifier);
> > +	group->dma_owner = DMA_OWNER_NONE;
> 
> 
> DMA_OWNER_NONE is also questionable. All devices are always in one
> domain, and the default domain is always the one used for DMA-API, so
> why isn't the initial value DMA_OWNER_DMA_API?

The interesting part is the suppress_auto_claim_dma_owner flag, but it
might make more sense to release the dma API ownership for that rather
than requesting it if it is not set.