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

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Message-ID: <20251203143609.00006b72@linux.microsoft.com>
Date: Wed, 3 Dec 2025 14:36:09 -0800
From: Jacob Pan <jacob.pan@...ux.microsoft.com>
To: 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>, Robin Murphy <robin.murphy@....com>, 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

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.