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:

quoted

On 11/11/25 13:12, Nicolin Chen wrote:

quoted

+/**
+ * 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/ (local)

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

quoted

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 ...

quoted

+ */
  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@nvidia.com/ (local)
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