iommu: Document usage of "/sys/kernel/iommu_groups/<grp_id>/type" file

The default domain type of an iommu group can be changed by writing to
"/sys/kernel/iommu_groups/<grp_id>/type" file. Hence, document it's usage
and more importantly spell out its limitations.
Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Sohil Mehta <sohil.mehta@intel.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Link: https://lore.kernel.org/r/20201124130604.2912899-5-baolu.lu@linux.intel.comSigned-off-by: Will Deacon <will@kernel.org>

Documentation/ABI/testing/sysfs-kernel-iommu_groups

@@ -33,3 +33,32 @@ Description:    In case an RMRR is used only by graphics or USB devices
 		it is now exposed as "direct-relaxable" instead of "direct".
 		In device assignment use case, for instance, those RMRR
 		are considered to be relaxable and safe.
+
+What:		/sys/kernel/iommu_groups/<grp_id>/type
+Date:		November 2020
+KernelVersion:	v5.11
+Contact:	Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
+Description:	/sys/kernel/iommu_groups/<grp_id>/type shows the type of default
+		domain in use by iommu for this group. See include/linux/iommu.h
+		for possible values. A privileged user could request kernel to
+		change the group type by writing to this file. Presently, only
+		three types of request are supported:
+		1. DMA: All the DMA transactions from the device in this group
+			are translated by the iommu.
+		2. identity: All the DMA transactions from the device in this
+			     group are *not* translated by the iommu.
+		3. auto: Change to the type the device was booted with.
+		Note:
+		-----
+		The default domain type of a group may be modified only when
+		1. The group has *only* one device
+		2. The device in the group is not bound to any device driver.
+		   So, the users must unbind the appropriate driver before
+		   changing the default domain type.
+		Caution:
+		--------
+		Unbinding a device driver will take away the driver's control
+		over the device and if done on devices that host root file
+		system could lead to catastrophic effects (the users might
+		need to reboot the machine to get it to normal state). So, it's
+		expected that the users understand what they're doing.