Re: [PATCH] iommu/arm-smmu-qcom: do not register driver in probe()

[Robin Murphy]

On 21/01/2026 2:12 pm, Danilo Krummrich wrote:
> Commit 0b4eeee2876f ("iommu/arm-smmu-qcom: Register the TBU driver in
> qcom_smmu_impl_init") intended to also probe the TBU driver when
> CONFIG_ARM_SMMU_QCOM_DEBUG is disabled, but also moved the corresponding
> platform_driver_register() call into qcom_smmu_impl_init() which is
> called from arm_smmu_device_probe().
>
> However, it neither makes sense to register drivers from probe()
> callbacks of other drivers, nor does the driver core allow registering
> drivers with a device lock already being held.
>
> The latter was revealed by commit dc23806a7c47 ("driver core: enforce
> device_lock for driver_match_device()") leading to a deadlock condition
> described in [1].
>
> Additionally, it was noted by Robin that the current approach is
> potentially racy with async probe [2].

On third look, the current lack of unregister, when it can end up in a 
module which (theoretically) supports unloading, is also sketchy :)

> Hence, fix this by registering the qcom_smmu_tbu_driver from
> module_init(). Unfortunately, due to the vendoring of the driver, this
> requires an indirection through arm-smmu-impl.c.
>
> Reported-by: Mark Brown <broonie@xxxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/7ae38e31-ef31-43ad-9106-7c76ea0e8596@xxxxxxxxxxxxx/
> Link: https://lore.kernel.org/lkml/DFU7CEPUSG9A.1KKGVW4HIPMSH@xxxxxxxxxx/ [1]
> Link: https://lore.kernel.org/lkml/0c0d3707-9ea5-44f9-88a1-a65c62e3df8d@xxxxxxx/ [2]
> Fixes: dc23806a7c47 ("driver core: enforce device_lock for driver_match_device()")
> Fixes: 0b4eeee2876f ("iommu/arm-smmu-qcom: Register the TBU driver in qcom_smmu_impl_init")
> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>

Looks OK to me, thanks Danilo!

Acked-by: Robin Murphy <robin.murphy@xxxxxxx>