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

[Wang Jiayue <akaieurus@gmail.com>]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain, Size: 6226 bytes --]

On Wed, Jan 21, 2026 at 10:12 PM Danilo Krummrich <dakr@kernel.org> 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@kernel.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")

Glad the backtrace helped identify the root cause.

I applied this patch to the qemu setup where I successfully reproduced the
boot hang earlier, and the deadlock is resolved.

Tested-by: Wang Jiayue <akaieurus@gmail.com>
Reviewed-by: Wang Jiayue <akaieurus@gmail.com>

> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  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@kernel.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
>