[PATCH v15 1/5] docs: driver-api: gpio: rpmsg gpio driver over rpmsg bus

[Shenwei Wang <shenwei.wang@oss.nxp.com>]

From: Shenwei Wang <shenwei.wang@nxp.com>

Describes the gpio rpmsg transport protocol over the rpmsg bus between
the remote system and Linux.

Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
---
 Documentation/driver-api/gpio/gpio-rpmsg.rst | 242 +++++++++++++++++++
 Documentation/driver-api/gpio/index.rst      |   1 +
 2 files changed, 243 insertions(+)
 create mode 100644 Documentation/driver-api/gpio/gpio-rpmsg.rst

diff --git a/Documentation/driver-api/gpio/gpio-rpmsg.rst b/Documentation/driver-api/gpio/gpio-rpmsg.rst
new file mode 100644
index 000000000000..9af75cc759ba
--- /dev/null
+++ b/Documentation/driver-api/gpio/gpio-rpmsg.rst
@@ -0,0 +1,242 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+GPIO RPMSG (Remote Processor Messaging) Protocol
+================================================
+
+The GPIO RPMSG transport protocol is used for communication and interaction
+with GPIO controllers on remote processors via the RPMSG bus.
+
+Message Format
+--------------
+
+The out message to the remote consists of a 8-byte packet with the
+following layout:
+
+.. code-block:: none
+
+   +------+------+------+------+------+------+------+------+
+   | 0x00 | 0x01 | 0x02 | 0x03 | 0x04 | 0x05 | 0x06 | 0x07 |
+   |     cmd     |    line     |           value           |
+   +------+------+------+------+------+------+------+------+
+
+- **cmd**: Little endian. Command type.
+
+- **line**: Little endian. The GPIO line (pin) index.
+
+- **value**: Little endian. See details in the command description below.
+
+
+The in message from the remote has the following layout:
+
+.. code-block:: none
+
+   +------+--------+--------+
+   | 0x00 |  0x01  |  0x02  |
+   | type |     payload     |
+   +------+--------+--------+
+
+- **type**: Message types.
+  - 0: Command reply messages.
+  - 1: Interrupt messages.
+
+- **payload**: See details below.
+
+GPIO Commands
+-------------
+
+Commands are specified in the **Cmd** field.
+
+The SEND message is always sent from Linux to the remote firmware. Each
+SEND corresponds to a single REPLY message. The GPIO driver should
+serialize messages and determine whether a REPLY message is required. If a
+REPLY message is expected but not received within the specified timeout
+period (currently 1 second in the Linux driver), the driver should return
+-ETIMEDOUT.
+
+GET_DIRECTION (Cmd=2)
+~~~~~~~~~~~~~~~~~~~~~
+
+**Request:**
+
+.. code-block:: none
+
+   +------+------+------+------+------+------+------+------+
+   | 0x00 | 0x01 | 0x02 | 0x03 | 0x04 | 0x05 | 0x06 | 0x07 |
+   |      2      |    line     |             0             |
+   +------+------+------+------+------+------+------+------+
+
+**Reply:**
+
+.. code-block:: none
+
+   +------+--------+--------+
+   | 0x00 |  0x01  |  0x02  |
+   |   0  | status | value  |
+   +------+--------+--------+
+
+- **status**:
+
+  - 0: Ok
+  - 1: Error
+
+- **value**: Direction.
+
+  - 0: None
+  - 1: Output
+  - 2: Input
+
+
+SET_DIRECTION (Cmd=3)
+~~~~~~~~~~~~~~~~~~~~~
+
+**Request:**
+
+.. code-block:: none
+
+   +------+------+------+------+------+------+------+------+
+   | 0x00 | 0x01 | 0x02 | 0x03 | 0x04 | 0x05 | 0x06 | 0x07 |
+   |      3      |    line     |           value           |
+   +------+------+------+------+------+------+------+------+
+
+- **value**: Direction.
+
+  - 0: None
+  - 1: Output
+  - 2: Input
+
+**Reply:**
+
+.. code-block:: none
+
+   +------+--------+--------+
+   | 0x00 |  0x01  |  0x02  |
+   |   0  | status |    0   |
+   +------+--------+--------+
+
+- **status**:
+
+  - 0: Ok
+  - 1: Error
+
+
+GET_VALUE (Cmd=4)
+~~~~~~~~~~~~~~~~~
+
+**Request:**
+
+.. code-block:: none
+
+   +------+------+------+------+------+------+------+------+
+   | 0x00 | 0x01 | 0x02 | 0x03 | 0x04 | 0x05 | 0x06 | 0x07 |
+   |      4      |    line     |             0             |
+   +------+------+------+------+------+------+------+------+
+
+**Reply:**
+
+.. code-block:: none
+
+   +------+--------+--------+
+   | 0x00 |  0x01  |  0x02  |
+   |   0  | status | value  |
+   +------+--------+--------+
+
+- **status**:
+
+  - 0: Ok
+  - 1: Error
+
+- **value**: Level.
+
+  - 0: Low
+  - 1: High
+
+
+SET_VALUE (Cmd=5)
+~~~~~~~~~~~~~~~~~
+
+**Request:**
+
+.. code-block:: none
+
+   +------+------+------+------+------+------+------+------+
+   | 0x00 | 0x01 | 0x02 | 0x03 | 0x04 | 0x05 | 0x06 | 0x07 |
+   |      5      |    line     |           value           |
+   +------+------+------+------+------+------+------+------+
+
+- **value**: Output level.
+
+  - 0: Low
+  - 1: High
+
+**Reply:**
+
+.. code-block:: none
+
+   +------+--------+--------+
+   | 0x00 |  0x01  |  0x02  |
+   |   0  | status |    0   |
+   +------+--------+--------+
+
+- **status**:
+
+  - 0: Ok
+  - 1: Error
+
+
+SET_IRQ_TYPE (Cmd=6)
+~~~~~~~~~~~~~~~~~~~~
+
+**Request:**
+
+.. code-block:: none
+
+   +------+------+------+------+------+------+------+------+
+   | 0x00 | 0x01 | 0x02 | 0x03 | 0x04 | 0x05 | 0x06 | 0x07 |
+   |      6      |    line     |           value           |
+   +------+------+------+------+------+------+------+------+
+
+- **value**: IRQ types.
+
+  - 0: Interrupt disabled
+  - 1: Rising edge trigger
+  - 2: Falling edge trigger
+  - 3: Both edge trigger
+  - 4: High level trigger
+  - 8: Low level trigger
+
+**Reply:**
+
+.. code-block:: none
+
+   +------+--------+--------+
+   | 0x00 |  0x01  |  0x02  |
+   |   0  | status |    0   |
+   +------+--------+--------+
+
+- **status**:
+
+  - 0: Ok
+  - 1: Error
+
+
+Interrupt Messages
+------------------
+
+Interrupt messages are sent by the remote core and they have
+**Type=1 (GPIO_RPMSG_NOTIFY)**:
+
+When a GPIO line asserts an interrupt on the remote processor, the firmware
+should immediately mask the corresponding interrupt source and send a
+notification message to the Linux. Upon completion of the interrupt
+handling on the Linux side, the driver should issue a
+command **SET_IRQ_TYPE** to the firmware to unmask the interrupt.
+
+.. code-block:: none
+
+   +------+------+--------+
+   | 0x00 | 0x01 |  0x02  |
+   |   1  |     line      |
+   +------+------+--------+
+
+- **line**: Little endian. The GPIO line (pin) index.
+
diff --git a/Documentation/driver-api/gpio/index.rst b/Documentation/driver-api/gpio/index.rst
index bee58f709b9a..e5eb1f82f01f 100644
--- a/Documentation/driver-api/gpio/index.rst
+++ b/Documentation/driver-api/gpio/index.rst
@@ -16,6 +16,7 @@ Contents:
    drivers-on-gpio
    bt8xxgpio
    pca953x
+   gpio-rpmsg
 
 Core
 ====
-- 
2.43.0