Re: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev_locked() helper

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Aug 11, 2025 at 03:59:10PM -0700, Nicolin Chen wrote:

> Introduce a new iommu_get_domain_for_dev_locked() helper to be used by
> those drivers in a context that is already under the protection of the
> group->mutex, e.g. those attach_dev callback functions. And roll out the
> new helper to all the existing IOMMU drivers.

I still don't much care for this, I think it would be better to
approach this problem by passing the old domain to the driver callback:

> @@ -390,7 +390,7 @@ static int qcom_iommu_attach_dev(struct iommu_domain *domain, struct device *dev
>  static int qcom_iommu_identity_attach(struct iommu_domain *identity_domain,
>  				      struct device *dev)
>  {
> -	struct iommu_domain *domain = iommu_get_domain_for_dev(dev);
> +	struct iommu_domain *domain = iommu_get_domain_for_dev_locked(dev);

Because this is a very common pattern in drivers.

Once that is done we can see what calls to iommu_get_domain_for_dev()
are even left, arguably we should be trying to eliminate this badly
locked thing...

Jason