Re: [PATCH v1 0/4] iommu: Fix device lookup lifetime and probe cleanup

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

On 14/07/2026 3:06 pm, Pranjal Shrivastava wrote:
> On Tue, Jul 14, 2026 at 02:09:26PM +0800, weimin xiong wrote:
>> Fix a few IOMMU driver lifetime and error-path issues found while
>> auditing fwnode-based device lookup and probe cleanup paths.
>>
>> The first three patches avoid deriving driver private data after
>> dropping the device reference returned by bus_find_device_by_fwnode().
>> They also make the ARM SMMU v2 and VSI probe paths fail cleanly when the
>> IOMMU lookup fails.
> 
> I'm not sure if that's really needed? All these drivers are doing is
> dropping the "extra" refcount (incremented by calling find_device)  back
> to the state *before* the fwnode function call. If you find that this
> put_device caused the count to drop to 0, I believe that's the real
> problem/bug. These fwnode functions are usually called in probe and the
> refcount shouldn't be 0 inside probe.
> 
> Could you share your observation / failing logs where this fails? Maybe
> something else is wrong with the system?

I don't have any trace of the original patches (thanks, Microsoft...) 
but looking on lore, yes these "lifetime" concerns are spurious; it's 
just a particular situation where due to the API, the drivers are taking 
a slightly roundabout route to look up their own valid device instance.

The IOMMU device must already have at least one held reference from way 
back in its device_initialise(), which will not be released unless and 
until device_unregister() is called (which is probably never for a 
non-hotpluggable platform device once it has been successfully created). 
If someone unregistered a platform device while it still had a driver 
bound, or the IOMMU driver could be unbound without unregistering the 
iommu_device through which its ->of_xlate or ->probe_device could be 
called, so many other things would be blowing up already that this would 
still be irrelevant.

Since 17de3f5fdd35 ("iommu: Retire bus ops") these lookups should also 
never return NULL for the same reasons, so do feel free to clean up 
those redundant checks if it helps make things a bit clearer.

Thanks,
Robin.