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Message-ID: <h2jbtahw5zcrn3rckul6ixesdvqpcdboz6daselk6cpnxosrii@ui45pvdammup>
Date: Wed, 21 Jan 2026 09:54:11 -0600
From: Bjorn Andersson <andersson@...nel.org>
To: Danilo Krummrich <dakr@...nel.org>
Cc: gregkh@...uxfoundation.org, 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>
Thank you Danilo.
Tested-by: Bjorn Andersson <andersson@...nel.org>
Reviewed-by: Bjorn Andersson <andersson@...nel.org>
Regards,
Bjorn
> ---
> drivers/iommu/arm/arm-smmu/arm-smmu-impl.c | 14 +++++++++++++
> drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 14 +++++++++----
> drivers/iommu/arm/arm-smmu/arm-smmu.c | 24 +++++++++++++++++++++-
> drivers/iommu/arm/arm-smmu/arm-smmu.h | 5 +++++
> 4 files changed, 52 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-impl.c b/drivers/iommu/arm/arm-smmu/arm-smmu-impl.c
> index db9b9a8e139c..4565a58bb213 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-impl.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-impl.c
> @@ -228,3 +228,17 @@ struct arm_smmu_device *arm_smmu_impl_init(struct arm_smmu_device *smmu)
>
> return smmu;
> }
> +
> +int __init arm_smmu_impl_module_init(void)
> +{
> + if (IS_ENABLED(CONFIG_ARM_SMMU_QCOM))
> + return qcom_smmu_module_init();
> +
> + return 0;
> +}
> +
> +void __exit arm_smmu_impl_module_exit(void)
> +{
> + if (IS_ENABLED(CONFIG_ARM_SMMU_QCOM))
> + qcom_smmu_module_exit();
> +}
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> index 573085349df3..22906d2c9a2d 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> @@ -774,10 +774,6 @@ struct arm_smmu_device *qcom_smmu_impl_init(struct arm_smmu_device *smmu)
> {
> const struct device_node *np = smmu->dev->of_node;
> const struct of_device_id *match;
> - static u8 tbu_registered;
> -
> - if (!tbu_registered++)
> - platform_driver_register(&qcom_smmu_tbu_driver);
>
> #ifdef CONFIG_ACPI
> if (np == NULL) {
> @@ -802,3 +798,13 @@ struct arm_smmu_device *qcom_smmu_impl_init(struct arm_smmu_device *smmu)
>
> return smmu;
> }
> +
> +int __init qcom_smmu_module_init(void)
> +{
> + return platform_driver_register(&qcom_smmu_tbu_driver);
> +}
> +
> +void __exit qcom_smmu_module_exit(void)
> +{
> + platform_driver_unregister(&qcom_smmu_tbu_driver);
> +}
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> index 5e690cf85ec9..1e218fbea35a 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> @@ -2365,7 +2365,29 @@ static struct platform_driver arm_smmu_driver = {
> .remove = arm_smmu_device_remove,
> .shutdown = arm_smmu_device_shutdown,
> };
> -module_platform_driver(arm_smmu_driver);
> +
> +static int __init arm_smmu_init(void)
> +{
> + int ret;
> +
> + ret = platform_driver_register(&arm_smmu_driver);
> + if (ret)
> + return ret;
> +
> + ret = arm_smmu_impl_module_init();
> + if (ret)
> + platform_driver_unregister(&arm_smmu_driver);
> +
> + return ret;
> +}
> +module_init(arm_smmu_init);
> +
> +static void __exit arm_smmu_exit(void)
> +{
> + arm_smmu_impl_module_exit();
> + platform_driver_unregister(&arm_smmu_driver);
> +}
> +module_exit(arm_smmu_exit);
>
> MODULE_DESCRIPTION("IOMMU API for ARM architected SMMU implementations");
> MODULE_AUTHOR("Will Deacon <will@...nel.org>");
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.h b/drivers/iommu/arm/arm-smmu/arm-smmu.h
> index 2dbf3243b5ad..26d2e33cd328 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.h
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.h
> @@ -540,6 +540,11 @@ struct arm_smmu_device *arm_smmu_impl_init(struct arm_smmu_device *smmu);
> struct arm_smmu_device *nvidia_smmu_impl_init(struct arm_smmu_device *smmu);
> struct arm_smmu_device *qcom_smmu_impl_init(struct arm_smmu_device *smmu);
>
> +int __init arm_smmu_impl_module_init(void);
> +void __exit arm_smmu_impl_module_exit(void);
> +int __init qcom_smmu_module_init(void);
> +void __exit qcom_smmu_module_exit(void);
> +
> void arm_smmu_write_context_bank(struct arm_smmu_device *smmu, int idx);
> int arm_mmu500_reset(struct arm_smmu_device *smmu);
>
>
> base-commit: dc23806a7c47ec5f1293aba407fb69519f976ee0
> --
> 2.52.0
>
>
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