A simple understanding of the relationship between Dev/iommu_group/iommu_domain and the three people

The struct Iommu_group *group can be obtained from the device structure;
For example:
struct Iommu_group *iommu_group_get (struct device *dev)
{
struct Iommu_group *group = dev->iommu_group;

if (group)
Kobject_get (Group->devices_kobj);

return group;
}
Group = Iommu_group_get (dev);
And Iommu_group has included Iommu_domain
For example: domain = group->domain;
To sum up: dev->iommu_group->domain. Visible domain represents a specific device using Iommu's detailed spec.
With Domian, only you can set up this dev using Smmu detailed spec by Iommu_dma_init_domain.
int Iommu_dma_init_domain (struct iommu_domain *domain, dma_addr_t base,
U64 size, struct device *dev)
{
struct Iova_domain *iovad = cookie_iovad (domain);
unsigned long order, BASE_PFN, END_PFN;

if (!iovad)
Return-enodev;

/* Use the smallest supported page size for Iova granularity * *
Order = __ffs (Domain->pgsize_bitmap);
BASE_PFN = max_t (unsigned long, 1, base >> order);
END_PFN = (base + size-1) >> order;
}
For example, we will decide the order and so on according to Domain->pgsize_bitmap. Therefore, domain represents this dev using Iommu detailed spec.
At present, the Iommu_group in the system is mainly divided into two groups, one is PCIe. One is generic, which can be seen from the following function
static struct Iommu_group *arm_smmu_device_group (struct device *dev)
{
struct Iommu_group *group;

/*
* We don ' t support devices sharing stream IDs other than PCI RIDs
* aliases, since the necessary Id-to-device lookup becomes rather
* Impractical given a potential sparse 32-bit stream ID space.
*/
if (Dev_is_pci (dev))
Group = Pci_device_group (dev);
Else
Group = Generic_device_group (dev);

return group;
}