The rk3288 has several iommus.  Each iommu belongs to a single master
device.  There is one device (ISP) that has two slave iommus, but that
case is not yet supported by this driver.

At subsys init, the iommu driver registers itself as the iommu driver for
the platform bus.  The master devices find their slave iommus using the
"iommus" field in their devicetree description.  Since each slave iommu
belongs to exactly one master, their is no additional data needed at probe
to associate a slave with its master.

An iommu device's power domain, clock and irq are all shared with its
master device, and the master device must be careful to attach from the
iommu only after powering and clocking it (and leave it powered and
clocked before detaching).  Because their is no guarantee what the status
of the iommu is at probe, and since the driver does not even know if the
device is powered, we delay requesting its irq until the master device
attaches, at which point we have a guarantee that the device is powered
and clocked and we can reset it and disable its interrupt mask.

An iommu_domain describes a virtual iova address space.  Each iommu_domain
has a corresponding page table that lists the mappings from iova to
physical address.

For the rk3288 iommu, the page table has two levels:
 The Level 1 "directory_table" has 1024 4-byte dte entries.
 Each dte points to a level 2 "page_table".
 Each level 2 page_table has 1024 4-byte pte entries.
 Each pte points to a 4 KiB page of memory.

An iommu_domain is created when a dma_iommu_mapping is created via
arm_iommu_create_mapping.  Master devices can then attach themselves to
this mapping (or attach the mapping to themselves?) by calling
arm_iommu_attach_device().  This in turn instructs the iommu driver to
write the page table's physical address into the slave iommu's "Directory
Table Entry" (DTE) register.

In fact multiple master devices, each with their own slave iommu device,
can all attach to the same mapping.  The iommus for these devices will
share the same iommu_domain and therefore point to the same page table.
Thus, the iommu domain maintains a list of iommu devices which are
attached.  This driver relies on the iommu core to ensure that all devices
have detached before destroying a domain.

v6: - add .add/remove_device() callbacks.
    - parse platform_device device tree nodes for "iommus" property
    - store platform device pointer as group iommudata
    - Check for existence of iommu group instead of relying on a
      dev_get_drvdata() to return NULL for a NULL device.

v7: - fixup some strings.
    - In rk_iommu_disable_paging() # and % were reversed.
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Simon Xue <xxm@rock-chips.com>
Reviewed-by: Grant Grundler <grundler@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>