[v13,3/4] docs: Add documentation for userspace client interface

diff --git a/Documentation/mhi/index.rst b/Documentation/mhi/index.rst
index 1d8dec3..c75a371 100644
--- a/Documentation/mhi/index.rst
+++ b/Documentation/mhi/index.rst
@@ -9,6 +9,7 @@  MHI
 
    mhi
    topology
+   uci
 
 .. only::  subproject and html
 
diff --git a/Documentation/mhi/uci.rst b/Documentation/mhi/uci.rst
new file mode 100644
index 0000000..ad22650
--- /dev/null
+++ b/Documentation/mhi/uci.rst
@@ -0,0 +1,94 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================
+Userspace Client Interface (UCI)
+=================================
+
+UCI driver enables userspace clients to communicate to external MHI devices
+like modem and WLAN. UCI driver probe creates standard character device file
+nodes for userspace clients to perform open, read, write, poll and release file
+operations. UCI device object represents UCI device file node which gets
+instantiated as part of MHI UCI driver probe. UCI channel object represents
+MHI uplink or downlink channel.
+
+Operations
+==========
+
+open
+----
+
+Instantiates UCI channel object and starts MHI channels to move it to running
+state. Inbound buffers are queued to downlink channel transfer ring. Every
+subsequent open() increments UCI device reference count as well as UCI channel
+reference count.
+
+read
+----
+
+When data transfer is completed on downlink channel, transfer ring element
+buffer is copied to pending list. Reader is unblocked and data is copied to
+userspace buffer. Transfer ring element buffer is queued back to downlink
+channel transfer ring.
+
+write
+-----
+
+Write buffer is queued to uplink channel transfer ring if ring is not full. Upon
+uplink transfer completion buffer is freed.
+
+poll
+----
+
+Returns EPOLLIN | EPOLLRDNORM mask if pending list has buffers to be read by
+userspace. Returns EPOLLOUT | EPOLLWRNORM mask if MHI uplink channel transfer
+ring is not empty. Returns EPOLLERR when UCI driver is removed.
+
+release
+-------
+
+Decrements UCI device reference count and UCI channel reference count upon last
+release(). UCI channel clean up is performed. MHI channel moves to disable
+state and inbound buffers are freed.
+
+Usage
+=====
+
+Device file node is created with format:-
+
+/dev/mhi_<mhi_device_name>
+
+mhi_device_name includes mhi controller name and the name of the MHI channel
+being used by MHI client in userspace to send or receive data using MHI
+protocol.
+
+There is a separate character device file node created for each channel
+specified in MHI device id table. MHI channels are statically defined by MHI
+specification. The list of supported channels is in the channel list variable
+of mhi_device_id table in UCI driver.
+
+Qualcomm MSM Interface(QMI) Channel
+-----------------------------------
+
+Qualcomm MSM Interface(QMI) is a modem control messaging protocol used to
+communicate between software components in the modem and other peripheral
+subsystems. QMI communication is of request/response type or an unsolicited
+event type. libqmi is userspace MHI client which communicates to a QMI service
+using UCI device. It sends a QMI request to a QMI service using MHI channel 14
+or 16. QMI response is received using MHI channel 15 or 17 respectively. libqmi
+is a glib-based library for talking to WWAN modems and devices which speaks QMI
+protocol. For more information about libqmi please refer
+https://www.freedesktop.org/wiki/Software/libqmi/
+
+Usage Example
+~~~~~~~~~~~~~
+
+QMI command to retrieve device mode
+$ sudo qmicli -d /dev/mhi_0000\:02\:00.0_QMI --dms-get-model
+[/dev/mhi_0000:02:00.0_QMI] Device model retrieved:
+    Model: 'FN980m'
+
+Other Use Cases
+---------------
+
+Getting MHI device specific diagnostics information to userspace MHI diagnostic
+client using DIAG channel 4 (Host to device) and 5 (Device to Host).