linux-kernel - Re: [PATCH v6 01/11] iommu: Add dma ownership management interfaces

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Message-ID: <31dc9d19-362d-4c1a-1483-cc7f525c6dd3@arm.com>
Date:   Thu, 24 Feb 2022 08:58:36 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.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 v6 01/11] iommu: Add dma ownership management interfaces

On 2022-02-24 05:29, Lu Baolu wrote:
> On 2/24/22 1:16 PM, Lu Baolu wrote:
>> Hi Robin and Jason,
>>
>> On 2/24/22 2:02 AM, Jason Gunthorpe wrote:
>>> On Wed, Feb 23, 2022 at 06:00:06PM +0000, Robin Murphy wrote:
>>>
>>>> ...and equivalently just set owner_cnt directly to 0 here. I don't 
>>>> see a
>>>> realistic use-case for any driver to claim the same group more than 
>>>> once,
>>>> and allowing it in the API just feels like opening up various potential
>>>> corners for things to get out of sync.
>>> I am Ok if we toss it out to get this merged, as there is no in-kernel
>>> user right now.
>>
>> So we don't need the owner pointer in the API anymore, right?
> 
> Oh, NO.
> 
> The owner token represents that the group has been claimed for user
> space access. And the default domain auto-attach policy will be changed
> accordingly.
> 
> So we still need this. Sorry for the noise.

Exactly. In fact we could almost go the other way, and rename owner_cnt 
to dma_api_users and make it mutually exclusive with owner being set, 
but that's really just cosmetic. It's understandable enough as-is that 
owner_cnt > 0 with owner == NULL represents implicit DMA API ownership.

Cheers,
Robin.