Re: [PATCH] iommu: Fix bypass of IOMMU readiness check for multi-IOMMU devices

[Robin Murphy <robin.murphy@arm.com>]

On 2026-04-02 12:59 pm, Jason Gunthorpe wrote:
> On Thu, Apr 02, 2026 at 02:25:54PM +0300, Tudor Ambarus wrote:
> 
>> I can probably track whether all instances are ready, and defer if any
>> is not ready, but then I'll force the iommu clients to use the sketchy
>> replay path, which seems like a bad idea, according to Robin's feedback.
> 
> I didn't think that was sketchy, it is part of the boot ordering
> system to ensure that the iommu driver(s) is probed before the client
> devices.
> 
> Half operating a device is definately going to get things into trouble
> with broken/incomplete domain attachments at least.

The Exynos driver itself is actually fine, and doing everything right. 
We'll never have a "half-configured" client device in IOMMU API terms 
currently - only once both instances are registered such that both 
of_xlate calls can succeed (one for each specifier in the client 
device's "iommus" property) will we proceed to calling probe_device, 
which will then work as normal.

The issue here is purely in the race-avoidance scheme within 
of_iommu_configure() itself, which hasn't accounted for the fact that 
when it's looping over multiple specifiers, they don't necessarily all 
target the same IOMMU node. And it's only during a window where the 
instance targeted by the first specifier happens to be registered 
already, and the second is currently in the middle of registering.

Thanks,
Robin.