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.

Cc: Christoph Hellwig <h...@lst.de>
Cc: Joerg Roedel <j...@8bytes.org>
Cc: Ashok Raj <ashok....@intel.com>
Cc: Will Deacon <will.dea...@arm.com>
Cc: Lu Baolu <baolu...@linux.intel.com>
Cc: Sohil Mehta <sohil.me...@intel.com>
Cc: Robin Murphy <robin.mur...@arm.com>
Cc: Jacob Pan <jacob.jun....@linux.intel.com>
Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prak...@intel.com>
---
 .../ABI/testing/sysfs-kernel-iommu_groups     | 30 +++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-kernel-iommu_groups 
b/Documentation/ABI/testing/sysfs-kernel-iommu_groups
index 017f5bc3920c..a498daffeb0c 100644
--- a/Documentation/ABI/testing/sysfs-kernel-iommu_groups
+++ b/Documentation/ABI/testing/sysfs-kernel-iommu_groups
@@ -33,3 +33,33 @@ 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:          June 2020
+KernelVersion: v5.8
+Contact:       Sai Praneeth Prakhya <sai.praneeth.prak...@intel.com>
+Description:   Let the user know the type of default domain in use by iommu
+               for this group. A privileged user could request kernel to change
+               the group type by writing to this file. Presently, only three
+               types 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. When the
+                        user reads the file he would never see "auto". This is
+                        just a write only value.
+               Note:
+               -----
+               A group type could 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 user must first unbind the appropriate driver and
+                  then change 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 user might
+               need to reboot the machine to get it to normal state). So, it's
+               expected that the user understands what he is doing.
-- 
2.19.1

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