linux-kernel - Re: [RFC 2/8] iommu: Add a helper to check if any iommu device is registered

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Message-ID: <0f3dad6d-b320-4d6d-a0b2-8919f3fca5e3@arm.com>
Date: Thu, 4 Dec 2025 10:53:36 +0000
From: Robin Murphy <robin.murphy@....com>
To: Jacob Pan <jacob.pan@...ux.microsoft.com>,
 Jason Gunthorpe <jgg@...dia.com>
Cc: Baolu Lu <baolu.lu@...ux.intel.com>, linux-kernel@...r.kernel.org,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 Alex Williamson <alex.williamson@...hat.com>, Joerg Roedel
 <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Nicolin Chen <nicolinc@...dia.com>, "Tian, Kevin" <kevin.tian@...el.com>,
 "Liu, Yi L" <yi.l.liu@...el.com>, skhawaja@...gle.com,
 pasha.tatashin@...een.com, Zhang Yu <zhangyu1@...ux.microsoft.com>,
 Jean Philippe-Brucker <jean-philippe@...aro.org>,
 David Matlack <dmatlack@...gle.com>, Alex Williamson <alex@...zbot.org>
Subject: Re: [RFC 2/8] iommu: Add a helper to check if any iommu device is
 registered

On 2025-12-03 10:36 pm, Jacob Pan wrote:
> Hi Jason,
> 
> On Wed, 3 Dec 2025 09:11:29 -0400
> Jason Gunthorpe <jgg@...dia.com> wrote:
> 
>> On Tue, Dec 02, 2025 at 04:06:35PM -0800, Jacob Pan wrote:
>>> However, as you pointed out there seems to be no standard ordering
>>> for iommu device registration across platforms. e.g. VT-d hooks up
>>> with x86_init, smmuv3 does that in platform driver probe. This
>>> patchset puts dummy driver under early_initcall which is after both
>>> but not a guarantee for all platforms. Any suggestions?
>>
>> I think we need to do something more like the sefltest does and
>> manually bind a driver to a device so this init time ordering
>> shouldn't matter.
> I have moved this dummy iommu driver init under iommufd_init(), which
> aligns well since it runs after all physical IOMMU drivers have
> registered. This dummy driver is intended for iommufd after all. But I
> don't see a need to bind to a platform device as the selttest does.

There is no "after all physical IOMMU drivers have registered", there is 
only "after we've given up waiting to see if one might be loaded as a 
module", but even that may be indefinite depending on build/runtime 
configuration.

Thanks,
Robin.