Re: [RFC 2/8] iommu: Add a helper to check if any iommu device is registered

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

On 12/3/25 08:06, Jacob Pan wrote:
> Hi Baolu,
> 
> On Tue, 2 Dec 2025 10:17:34 +0800
> Baolu Lu <baolu.lu@linux.intel.com> wrote:
> 
>> Hi Jacob,
>>
>> On 12/2/25 01:30, Jacob Pan wrote:
>>> The dummy IOMMU driver for No-IOMMU mode should only be active when
>>> no real IOMMU devices are present in the system. Introduce a helper
>>> to check this condition, ensuring that the dummy driver does not
>>> interfere when hardware-backed IOMMU support is available.
>>>
>>> Signed-off-by: Jacob Pan<jacob.pan@linux.microsoft.com>
>>> ---
>>>    drivers/iommu/iommu.c | 10 ++++++++++
>>>    include/linux/iommu.h |  1 +
>>>    2 files changed, 11 insertions(+)
>>>
>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>>> index 0df914a04064..958f612bf176 100644
>>> --- a/drivers/iommu/iommu.c
>>> +++ b/drivers/iommu/iommu.c
>>> @@ -2895,6 +2895,16 @@ static const struct iommu_device
>>> *iommu_from_fwnode(const struct fwnode_handle * return ret;
>>>    }
>>>    
>>> +bool iommu_is_registered(void)
>>> +{
>>> +	bool registered;
>>> +
>>> +	spin_lock(&iommu_device_lock);
>>> +	registered = !list_empty(&iommu_device_list);
>>> +	spin_unlock(&iommu_device_lock);
>>> +	return registered;
>>> +}
>>
>> IOMMU devices might be added by calling iommu_device_register() at any
>> time. Therefore, an empty iommu_device_list does not necessarily mean
>> that "no real IOMMU devices are present in the system."
> Good point. My intention was that the noiommu dummy driver should
> register only after all hardware IOMMU drivers have completed
> registration during boot. Any subsequent registration attempt, such as a
> hot-added IOMMU, should fail if noiommu mode is already active.
> 
> We could enforce this by introducing a global flag that prevents any
> iommu_device from being registered after the noiommu driver has been
> initialized.
> 
> However, as you pointed out there seems to be no standard ordering
> for iommu device registration across platforms. e.g. VT-d hooks up with
> x86_init, smmuv3 does that in platform driver probe. This patchset puts
> dummy driver under early_initcall which is after both but not a
> guarantee for all platforms. Any suggestions?

Could we add a helper call inside the IOMMU detection path (e.g., within
pci_iommu_alloc()) to set a flag, such as platform_iommu_present, after
successful hardware detection?

void __init pci_iommu_alloc(void)
{
         if (xen_pv_domain()) {
                 pci_xen_swiotlb_init();
                 return;
         }
         pci_swiotlb_detect();
         gart_iommu_hole_init();
         amd_iommu_detect();
         detect_intel_iommu();
         swiotlb_init(x86_swiotlb_enable, x86_swiotlb_flags);
}

Thanks,
baolu