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

[Baolu Lu]

On 11/18/25 15:02, Nicolin Chen wrote:
> On Wed, Nov 12, 2025 at 09:41:25AM -0800, Nicolin Chen wrote:
> > 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@xxxxxxxxxx/
> >
> > 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?
>
> Hi Baolu,
>
> For v6, I tend to keep this API as-is, trying not to give troubles
> to existing callers. Jason suggested a potential followup series:
> https://lore.kernel.org/linux-iommu/20250821131304.GM802098@xxxxxxxxxx/
> That would replace this function, so maybe we can think about that.
>
> If you have a strong feeling about the WARN_ON, please let me know.
>
> Thanks
> Nicolin

No strong feeling. I am fine with it because the comments have already
stated that "This function can be called within a driver bound to dev.".

Thanks,
baolu