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

[Baolu Lu <baolu.lu@linux.intel.com>]

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@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?
> 
> 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/
> 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