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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.com
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Sai Praneeth Prakhya 2020-11-24 21:06:04 +08:00 committed by Will Deacon
parent 0b8a96a312
commit 63a816749d
1 changed files with 29 additions and 0 deletions
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29
Documentation/ABI/testing/sysfs-kernel-iommu_groups
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@ -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.