docs: infiniband: document the UCAP API

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>

Author: chiarameiohas

Documentation/infiniband/index.rst

@@ -12,6 +12,7 @@ InfiniBand
    opa_vnic
    sysfs
    tag_matching
+   ucaps
    user_mad
    user_verbs
 

Documentation/infiniband/ucaps.rst

@@ -0,0 +1,71 @@
+=================================
+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
+