Re: [PATCH v5 3/5] iommu: Add iommu_driver_get_domain_for_dev() helper

[Nicolin Chen <nicolinc@nvidia.com>]

Hi Baolu,

On Wed, Nov 12, 2025 at 01:58:51PM +0800, Baolu Lu wrote:
> On 11/11/25 13:12, Nicolin Chen wrote:
> > +/**
> > + * iommu_get_domain_for_dev() - Return the DMA API domain pointer
> > + * @dev - Device to query
> > + *
> > + * This function can be called within a driver bound to dev. The returned
> > + * pointer is valid for the lifetime of the bound driver.
> > + *
> > + * It should not be called by drivers with driver_managed_dma = true.
> 
> "driver_managed_dma != true" means the driver will use the default
> domain allocated by the iommu core during iommu probe.

Hmm, I am not very sure. Jason's remarks pointed out that There
is an exception in host1x_client_iommu_detach():
https://lore.kernel.org/all/20250924191055.GJ2617119@nvidia.com/

Where the group->domain could be NULL, i.e. not attached to the
default domain?

> The iommu core
> ensures that this domain stored at group->domain will not be changed
> during the driver's whole lifecycle. That's reasonable.
> 
> How about making some code to enforce this requirement? Something like
> below ...
> 
> > + */
> >   struct iommu_domain *iommu_get_domain_for_dev(struct device *dev)
> >   {
> >   	/* Caller must be a probed driver on dev */
> > @@ -2225,10 +2234,29 @@ struct iommu_domain *iommu_get_domain_for_dev(struct device *dev)
> >   	if (!group)
> >   		return NULL;
> > +	lockdep_assert_not_held(&group->mutex);
> 
> ...
> 	if (WARN_ON(!dev->driver || !group->owner_cnt || group->owner))
> 		return NULL;

With that, could host1x_client_iommu_detach() trigger WARN_ON?

Thanks!
Nicolin