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

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

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."

Thanks,
baolu