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

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Message-ID: <2026012139-crewmate-radiantly-d533@gregkh>
Date: Wed, 21 Jan 2026 15:46:49 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Danilo Krummrich <dakr@...nel.org>
Cc: rafael@...nel.org, broonie@...nel.org, will@...nel.org,
	robin.murphy@....com, joro@...tes.org, robin.clark@....qualcomm.com,
	hanguidong02@...il.com, quic_c_gdjako@...cinc.com,
	dmitry.baryshkov@....qualcomm.com, driver-core@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	iommu@...ts.linux.dev, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu-qcom: do not register driver in probe()

On Wed, Jan 21, 2026 at 03:12:01PM +0100, 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].
> 
> 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@...nel.org>
> Closes: https://lore.kernel.org/lkml/7ae38e31-ef31-43ad-9106-7c76ea0e8596@sirena.org.uk/
> Link: https://lore.kernel.org/lkml/DFU7CEPUSG9A.1KKGVW4HIPMSH@kernel.org/ [1]
> Link: https://lore.kernel.org/lkml/0c0d3707-9ea5-44f9-88a1-a65c62e3df8d@arm.com/ [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@...nel.org>

Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>