The default domain type of an iommu group can be changed using file
"/sys/kernel/iommu_groups/<grp_id>/type". Hence, document it's usage and
more importantly spell out it's limitations and an example.
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 | 34 +++++++++++++++++++
1 file changed, 34 insertions(+)
diff --git a/Documentation/ABI/testing/sysfs-kernel-iommu_groups
b/Documentation/ABI/testing/sysfs-kernel-iommu_groups
index 017f5bc3920c..7eb1b784c5e3 100644
--- a/Documentation/ABI/testing/sysfs-kernel-iommu_groups
+++ b/Documentation/ABI/testing/sysfs-kernel-iommu_groups
@@ -33,3 +33,37 @@ 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: July 2019
+KernelVersion: v5.4
+Contact: Sai Praneeth Prakhya <sai.praneeth.prak...@intel.com>
+Description: /sys/kernel/iommu_groups/<grp_id>/type lets 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 two types are supported
+ 1. dma: All the DMA transactions from the devices in this group
+ are translated by the iommu.
+ 2. identity: All the DMA transactions from the devices in this
+ group are *not* translated by the iommu.
+ Note:
+ -----
+ A group type could be modified only when *all* the devices in
+ the group are not binded to any device driver. So, the user has
+ to first unbind the appropriate drivers and then change the
+ default domain type.
+ Caution:
+ --------
+ Unbinding a device driver will take away the drivers 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.
+ Example:
+ --------
+ # Unbind USB device driver
+ 1. echo "0000:00:14.0" > /sys/bus/pci/drivers/xhci_hcd/unbind
+ # Put the USB device in identity mode (a.k.a pass through)
+ 2. echo "identity" > /sys/kernel/iommu_groups/<grp_id>/type
+ # Re-bind the driver
+ 3. echo "0000:00:14.0" > /sys/bus/pci/drivers/xhci_hcd/bind
--
2.19.1
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