Re: [PATCH] PCI: Disable ATS via quirk before notifying IOMMU drivers

[David Matlack]

On Wed, Feb 25, 2026 at 1:43 AM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> On Tue, Feb 24, 2026 at 09:41:30AM -0800, David Matlack wrote:
> > On Tue, Feb 24, 2026 at 9:25 AM Andy Shevchenko
> > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > On Tue, Feb 24, 2026 at 09:19:05AM -0800, David Matlack wrote:
> > > > On Mon, Feb 23, 2026 at 12:37 PM Andy Shevchenko
> > > > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > > > On Mon, Feb 23, 2026 at 06:40:16PM +0000, David Matlack wrote:
>
> ...
>
> > > > > > Fix this by disabling ATS via quirk during "early" fixups instead of
> > > > > > "final" fixups.
> > > > >
> > > > > Hmm... Sounds to me like a premature disablement, but I leave it the experts.
> > > >
> > > > What do you mean by "premature disablement"?
> > >
> > > On early stage instead of final stage.
> >
> > Is your concern that applying the quirk at early stage won't be
> > effective because ATS will be enabled after early fixups are applied?
>
> My concern that applying this quirk too early may affect something else.
> But I'm not an expert in the PCI mysterious ways, I just share my feelings.

Ahh ok, gotcha. Thanks for the raising the concern.

> > > > > What I think about the case, that IOMMU should be probably fixed to avoid such
> > > > > situation for all level of quirks. Can it be feasible?
> > > >
> > > > What do you mean by the "IOMMU should be fixed"? Are you saying the
> > > > IOMMU should be prepared to handle quirks disabling features on
> > > > devices after the IOMMU driver is notified about a device?
> > >
> > > Something like this, yes. At least the commit message is unclear why
> > > "This fixes at least one bug in the Intel IOMMU driver..." not in IOMMU
> > > driver code.
> >
> > Gotcha. It felt wrong to have the IOMMU driver be notified about a
> > device with ATS enabled and then have ATS later disabled. It seem like
> > it would add complexity to the IOMMU drivers to handle such a case,
> > and would be much simpler to have ATS in its final state when the
> > IOMMU driver is notified about the device being created.
>
> Can this be elaborated in the commit message? Then a reviewer will not
> have questions like me.

Will do in v2, thanks for the suggestion.