[PATCH v6 3/4] docs: Add documentation for userspace client interface

[Hemant Kumar <hemantk@codeaurora.org>]

MHI userspace client driver is creating device file node
for user application to perform file operations. File
operations are handled by MHI core driver. Currently
Loopback MHI channel is supported by this driver.

Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
---
 Documentation/mhi/index.rst |  1 +
 Documentation/mhi/uci.rst   | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)
 create mode 100644 Documentation/mhi/uci.rst

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..5d92939
--- /dev/null
+++ b/Documentation/mhi/uci.rst
@@ -0,0 +1,39 @@
+.. 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. It creates standard character device file nodes for user
+space clients to perform open, read, write, poll and close file operations.
+
+Device file node is created with format:-
+
+/dev/mhi_<controller_name>_<mhi_device_name>
+
+controller_name is the name of underlying bus used to transfer data.
+mhi_device_name is 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.
+Driver currently supports LOOPBACK channel 0 (Host to device) and 1 (Device to Host).
+
+LOOPBACK Channel
+----------------
+
+Userspace MHI client using LOOPBACK channel opens device file node. As part of
+open operation TREs to transfer ring of LOOPBACK channel 1 gets queued and channel
+doorbell is rung. When userspace MHI client performs write operation on device node,
+data buffer gets queued as a TRE to transfer ring of LOOPBACK channel 0. MHI Core
+driver rings the channel doorbell for MHI device to move data over underlying bus.
+When userspace MHI client driver performs read operation, same data gets looped back
+to MHI host using LOOPBACK channel 1. LOOPBACK channel is used to verify data path
+and data integrity between MHI Host and MHI device.
+
+Other Use Cases
+---------------
+
+Getting MHI device specific diagnostics information to userspace MHI diag client
+using DIAG channel 4 (Host to device) and 5 (Device to Host).
-- 
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