Describes the gpio rpmsg transport protocol over the rpmsg bus between
the cores.

Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
---
 Documentation/driver-api/gpio/gpio-rpmsg.rst | 232 +++++++++++++++++++
 Documentation/driver-api/gpio/index.rst      |   1 +
 2 files changed, 233 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..c78d10a9a85c
--- /dev/null
+++ b/Documentation/driver-api/gpio/gpio-rpmsg.rst
@@ -0,0 +1,232 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+GPIO RPMSG Protocol
+===================
+
+The GPIO RPMSG transport protocol is used for communication and interaction
+with GPIO controllers located on remote cores on the RPMSG bus.
+
+Message Format
+--------------
+
+The RPMSG message consists of a 14-byte packet with the following layout:
+
+.. code-block:: none
+
+   +-----+-------+--------+-----+-----+------------+-----+-----+-----+----+
+   |0x00 |0x01   |0x02    |0x03 |0x04 |0x05..0x09  |0x0A |0x0B |0x0C |0x0D|
+   | ID  |vendor |version |type |cmd  |reserved[5] |line |port |  data    |
+   +-----+-------+--------+-----+-----+------------+-----+-----+-----+----+
+
+- **ID (Message Identification Code)**: Always be 0x5. Indicates the GPIO message.
+
+- **Vendor**: Vendor ID number.
+  - 0: Reserved
+  - 1: NXP
+
+- **Version**: Vendor-specific version number (such as software release).
+
+- **Type (Message Type)**: The message type can be one of:
+
+  - 0: GPIO_RPMSG_SETUP
+  - 1: GPIO_RPMSG_REPLY
+  - 2: GPIO_RPMSG_NOTIFY
+
+- **Cmd**: Command code, used for GPIO_RPMSG_SETUP messages.
+
+- **reserved[5]**: Reserved bytes. Should always be 0.
+
+- **line**: The GPIO line index.
+
+- **port**: The GPIO controller index.
+
+- **data**: See details in the command description below.
+
+GPIO Commands
+-------------
+
+Commands are specified in the **Cmd** field for **GPIO_RPMSG_SETUP** (Type=0) messages.
+
+The SETUP message is always sent from Linux to the remote firmware. Each
+SETUP 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
+-ETIMEOUT.
+
+GPIO_RPMSG_INPUT_INIT (Cmd=0)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**Request:**
+
+.. code-block:: none
+
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05..0x09 |0x0A |0x0B |0x0C |0x0D|
+   | 5   | 1   | 0   | 0   | 0   |  0        |line |port | val | wk |
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+
+- **val**: Interrupt trigger type.
+
+  - 0: Interrupt disabled
+  - 1: Rising edge trigger
+  - 2: Falling edge trigger
+  - 3: Both edge trigger
+  - 4: Low level trigger
+  - 5: High level trigger
+
+- **wk**: Wakeup enable.
+
+  The remote system should always aim to stay in a power-efficient state by
+  shutting down or clock-gating the GPIO blocks that aren't in use. Since
+  the remoteproc driver is responsibe for managing the power states of the
+  remote firmware, the GPIO driver does not require to konow the firmware's
+  running states.
+
+  When the wakeup bit is set, the remote firmware should configure the line
+  as a wakeup source. The firmware should send the notification message to
+  Linux after it is woken from the GPIO line.
+
+  - 0: Disable wakeup from GPIO
+  - 1: Enable wakeup from GPIO
+
+**Reply:**
+
+.. code-block:: none
+
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05..0x09 |0x0A |0x0B |0x0C |0x0D|
+   | 5   | 1   | 0   | 1   | 1   |  0        |line |port | err | 0  |
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+
+- **err**: Error code from the remote core.
+
+  - 0: Success
+  - 1: General error (early remote software only returns this unclassified error)
+  - 2: Not supported (A command is not supported by the remote firmware)
+  - 3: Resource not available (The resource is not allocated to the Linux)
+  - 4: Resource busy (The resource is already used)
+  - 5: Parameter error
+
+GPIO_RPMSG_OUTPUT_INIT (Cmd=1)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**Request:**
+
+.. code-block:: none
+
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05..0x09 |0x0A |0x0B |0x0C |0x0D|
+   | 5   | 1   | 0   | 0   | 1   |  0        |line |port | val | 0  |
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+
+- **val**: Output level.
+
+  - 0: Low
+  - 1: High
+
+**Reply:**
+
+.. code-block:: none
+
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05..0x09 |0x0A |0x0B |0x0C |0x0D|
+   | 5   | 1   | 0   | 1   | 1   |  0        |line |port | err | 0  |
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+
+- **err**: See above for definitions.
+
+GPIO_RPMSG_INPUT_GET (Cmd=2)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**Request:**
+
+.. code-block:: none
+
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05..0x09 |0x0A |0x0B |0x0C |0x0D|
+   | 5   | 1   | 0   | 0   | 2   |  0        |line |port | 0   | 0  |
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+
+**Reply:**
+
+.. code-block:: none
+
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+-----+
+   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05..0x09 |0x0A |0x0B |0x0C |0x0D |
+   | 5   | 1   | 0   | 1   | 2   |  0        |line |port | err |level|
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+-----+
+
+- **err**: See above for definitions.
+
+- **level**: Input level.
+
+  - 0: Low
+  - 1: High
+
+GPIO_RPMSG_GET_DIRECTION (Cmd=3)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**Request:**
+
+.. code-block:: none
+
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05..0x09 |0x0A |0x0B |0x0C |0x0D|
+   | 5   | 1   | 0   | 0   | 3   |  0        |line |port | 0   | 0  |
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+
+**Reply:**
+
+.. code-block:: none
+
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+-----+
+   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05..0x09 |0x0A |0x0B |0x0C |0x0D |
+   | 5   | 1   | 0   | 1   | 3   |  0        |line |port | err | dir |
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+-----+
+
+- **err**: See above for definitions.
+
+- **dir**: Direction.
+
+  - 0: Output
+  - 1: Input
+
+Notification Message
+--------------------
+
+Notifications are sent with **Type=2 (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
+**GPIO_RPMSG_INPUT_INIT** command to the firmware to unmask the interrupt.
+
+A Notification message can arrive between a SETUP and its REPLY message,
+and the driver is expected to handle this scenario.
+
+.. code-block:: none
+
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05..0x09 |0x0A |0x0B |0x0C |0x0D|
+   | 5   | 1   | 0   | 2   | 0   |  0        |line |port | 0   | 0  |
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+
+- **line**: The GPIO line index.
+- **port**: The GPIO controller index.
+
+The reply message for the notification is optional. The remote firmware can
+implement it to simulate the interrupt acknowledgment behavior.
+
+The notification reply is sent with the byte index 0x4=1.
+
+.. code-block:: none
+
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05..0x09 |0x0A |0x0B |0x0C |0x0D|
+   | 5   | 1   | 0   | 2   | 1   |  0        |line |port | 0   | 0  |
+   +-----+-----+-----+-----+-----+-----------+-----+-----+-----+----+
+
+- **line**: The GPIO line index.
+- **port**: The GPIO controller 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