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docs: infiniband: document the UCAP API

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Add an explanation on the newly added UCAP API.

Signed-off-by: Chiara Meiohas <cmeiohas@nvidia.com>
Link: https://patch.msgid.link/d0e095f9a7601437acc2d2fdf8705136d1edf1c5.1741261611.git.leon@kernel.org
Reviewed-by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
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Chiara Meiohas 2025-03-06 13:51:31 +02:00 committed by Leon Romanovsky
parent 74934ddf12
commit 8820965c48
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Documentation/infiniband/index.rst
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@ -12,6 +12,7 @@ InfiniBand
opa_vnic opa_vnic
sysfs sysfs
tag_matching tag_matching
ucaps
user_mad user_mad
user_verbs user_verbs

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Documentation/infiniband/ucaps.rst Normal file
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=================================
Infiniband Userspace Capabilities
=================================
User CAPabilities (UCAPs) provide fine-grained control over specific
firmware features in Infiniband (IB) devices. This approach offers
more granular capabilities than the existing Linux capabilities,
which may be too generic for certain FW features.
Each user capability is represented as a character device with root
read-write access. Root processes can grant users special privileges
by allowing access to these character devices (e.g., using chown).
Usage
=====
UCAPs allow control over specific features of an IB device using file
descriptors of UCAP character devices. Here is how a user enables
specific features of an IB device:
* A root process grants the user access to the UCAP files that
represents the capabilities (e.g., using chown).
* The user opens the UCAP files, obtaining file descriptors.
* When opening an IB device, include an array of the UCAP file
descriptors as an attribute.
* The ib_uverbs driver recognizes the UCAP file descriptors and enables
the corresponding capabilities for the IB device.
Creating UCAPs
==============
To create a new UCAP, drivers must first define a type in the
rdma_user_cap enum in rdma/ib_ucaps.h. The name of the UCAP character
device should be added to the ucap_names array in
drivers/infiniband/core/ucaps.c. Then, the driver can create the UCAP
character device by calling the ib_create_ucap API with the UCAP
type.
A reference count is stored for each UCAP to track creations and
removals of the UCAP device. If multiple creation calls are made with
the same type (e.g., for two IB devices), the UCAP character device
is created during the first call and subsequent calls increment the
reference count.
The UCAP character device is created under /dev/infiniband, and its
permissions are set to allow root read and write access only.
Removing UCAPs
==============
Each removal decrements the reference count of the UCAP. The UCAP
character device is removed from the filesystem only when the
reference count is decreased to 0.
/dev and /sys/class files
=========================
The class::
/sys/class/infiniband_ucaps
is created when the first UCAP character device is created.
The UCAP character device is created under /dev/infiniband.
For example, if mlx5_ib adds the rdma_user_cap
RDMA_UCAP_MLX5_CTRL_LOCAL with name "mlx5_perm_ctrl_local", this will
create the device node::
/dev/infiniband/mlx5_perm_ctrl_local