[PATCH v6 12/12] Documentation: tee: Add Qualcomm TEE driver

[Amirreza Zarrabi <amirreza.zarrabi@oss.qualcomm.com>]

Add documentation for the Qualcomm TEE driver.

Signed-off-by: Amirreza Zarrabi <amirreza.zarrabi@oss.qualcomm.com>
---
 Documentation/tee/index.rst |  1 +
 Documentation/tee/qtee.rst  | 96 +++++++++++++++++++++++++++++++++++++++++++++
 MAINTAINERS                 |  1 +
 3 files changed, 98 insertions(+)

diff --git a/Documentation/tee/index.rst b/Documentation/tee/index.rst
index 4be6e69d7837..62afb7ee9b52 100644
--- a/Documentation/tee/index.rst
+++ b/Documentation/tee/index.rst
@@ -11,6 +11,7 @@ TEE Subsystem
    op-tee
    amd-tee
    ts-tee
+   qtee
 
 .. only::  subproject and html
 
diff --git a/Documentation/tee/qtee.rst b/Documentation/tee/qtee.rst
new file mode 100644
index 000000000000..2fa2c1bf6384
--- /dev/null
+++ b/Documentation/tee/qtee.rst
@@ -0,0 +1,96 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============================================
+QTEE (Qualcomm Trusted Execution Environment)
+=============================================
+
+The QTEE driver handles communication with Qualcomm TEE [1].
+
+The lowest level of communication with QTEE builds on the ARM SMC Calling
+Convention (SMCCC) [2], which is the foundation for QTEE's Secure Channel
+Manager (SCM) [3] used internally by the driver.
+
+In a QTEE-based system, services are represented as objects with a series of
+operations that can be called to produce results, including other objects.
+
+When an object is hosted within QTEE, executing its operations is referred
+to as "direct invocation". QTEE can also invoke objects hosted in the non-secure
+world using a method known as "callback request".
+
+The SCM provides two functions to support direct invocation and callback requests:
+
+- QCOM_SCM_SMCINVOKE_INVOKE: Used for direct invocation. It can return either
+  a result or initiate a callback request.
+- QCOM_SCM_SMCINVOKE_CB_RSP: Used to submit a response to a callback request
+  triggered by a previous direct invocation.
+
+The QTEE Transport Message [4] is stacked on top of the SCM driver functions.
+
+A message consists of two buffers shared with QTEE: inbound and outbound
+buffers. The inbound buffer is used for direct invocation, and the outbound
+buffer is used to make callback requests. This picture shows the contents of
+a QTEE transport message::
+
+                                      +---------------------+
+                                      |                     v
+    +-----------------+-------+-------+------+--------------------------+
+    | qcomtee_msg_    |object | buffer       |                          |
+    |  object_invoke  |  id   | offset, size |                          | (inbound buffer)
+    +-----------------+-------+--------------+--------------------------+
+    <---- header -----><---- arguments ------><- in/out buffer payload ->
+
+                                      +-----------+
+                                      |           v
+    +-----------------+-------+-------+------+----------------------+
+    | qcomtee_msg_    |object | buffer       |                      |
+    |  callback       |  id   | offset, size |                      | (outbound buffer)
+    +-----------------+-------+--------------+----------------------+
+
+Each buffer is started with a header and array of arguments.
+
+QTEE Transport Message supports four types of arguments:
+
+- Input Object (IO) is an object parameter to the current invocation
+  or callback request.
+- Output Object (OO) is an object parameter from the current invocation
+  or callback request.
+- Input Buffer (IB) is (offset, size) pair to the inbound or outbound region
+  to store parameter to the current invocation or callback request.
+- Output Buffer (OB) is (offset, size) pair to the inbound or outbound region
+  to store parameter from the current invocation or callback request.
+
+Picture of the relationship between the different components in the QTEE
+architecture::
+
+         User space               Kernel                     Secure world
+         ~~~~~~~~~~               ~~~~~~                     ~~~~~~~~~~~~
+   +--------+   +----------+                                +--------------+
+   | Client |   |callback  |                                | Trusted      |
+   +--------+   |server    |                                | Application  |
+      /\        +----------+                                +--------------+
+      ||  +----------+ /\                                          /\
+      ||  |callback  | ||                                          ||
+      ||  |server    | ||                                          \/
+      ||  +----------+ ||                                   +--------------+
+      ||       /\      ||                                   | TEE Internal |
+      ||       ||      ||                                   | API          |
+      \/       \/      \/   +--------+--------+             +--------------+
+   +---------------------+  | TEE    | QTEE   |             | QTEE         |
+   |   libqcomtee [5]    |  | subsys | driver |             | Trusted OS   |
+   +-------+-------------+--+----+-------+----+-------------+--------------+
+   |      Generic TEE API        |       |   QTEE MSG                      |
+   |      IOCTL (TEE_IOC_*)      |       |   SMCCC (QCOM_SCM_SMCINVOKE_*)  |
+   +-----------------------------+       +---------------------------------+
+
+References
+==========
+
+[1] https://docs.qualcomm.com/bundle/publicresource/topics/80-70015-11/qualcomm-trusted-execution-environment.html
+
+[2] http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html
+
+[3] drivers/firmware/qcom/qcom_scm.c
+
+[4] drivers/tee/qcomtee/qcomtee_msg.h
+
+[5] https://github.com/quic/quic-teec
diff --git a/MAINTAINERS b/MAINTAINERS
index 9fc58f48fa14..391fc6e6defc 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -20706,6 +20706,7 @@ QUALCOMM TEE (QCOMTEE) DRIVER
 M:	Amirreza Zarrabi <amirreza.zarrabi@oss.qualcomm.com>
 L:	linux-arm-msm@vger.kernel.org
 S:	Maintained
+F:	Documentation/tee/qtee.rst
 F:	drivers/tee/qcomtee/
 
 QUALCOMM TRUST ZONE MEMORY ALLOCATOR

-- 
2.34.1