[nrf fromtree] doc: ipc: Add detailed protocol specs for ICMsg backend

The details of how the ICMsg communication is done, are hidden
inside the source code. This specification adds details about
the ICBMsg protocol.

Signed-off-by: Dominik Kilian <[email protected]>

(cherry picked from commit 7574602)

Author: doki-nordic

doc/services/ipc/ipc_service/backends/ipc_service_icmsg.rst

@@ -84,3 +84,111 @@ Samples
 =======
 
  - :zephyr:code-sample:`ipc-icmsg`
+
+Detailed Protocol Specification
+===============================
+
+The ICMsg uses two shared memory regions and two MBOX channels.
+The region and channel pair are used to transfer messages in one direction.
+The other pair is symmetric and transfers messages in the opposite direction.
+For this reason, the specification below focuses on one such pair.
+The other pair is identical.
+
+The ICMsg provides just one endpoint per instance.
+
+Shared Memory Region Organization
+---------------------------------
+
+If data caching is enabled, the shared memory region provided to ICMsg must be aligned according to the cache requirement.
+If cache is not enabled, the required alignment is 4 bytes.
+
+The shared memory region is entirely used by a single FIFO.
+It contains read and write indexes followed by the data buffer.
+The detailed structure is contained in the following table:
+
+.. list-table::
+   :header-rows: 1
+
+   * - Field name
+     - Size (bytes)
+     - Byte order
+     - Description
+   * - ``rd_idx``
+     - 4
+     - little‑endian
+     - Index of the first incoming byte in the ``data`` field.
+   * - ``padding``
+     - depends on cache alignment
+     - n/a
+     - Padding added to align ``wr_idx`` to the cache alignment.
+   * - ``wr_idx``
+     - 4
+     - little‑endian
+     - Index of the byte after the last incoming byte in the ``data`` field.
+   * - ``data``
+     - everything to the end of the region
+     - n/a
+     - Circular buffer containing actual bytes to transfer.
+
+This is usual FIFO with a circular buffer:
+
+* The Indexes (``rd_idx`` and ``wr_idx``) are wrapped around when they reach the end of the ``data`` buffer.
+* The FIFO is empty if ``rd_idx == wr_idx``.
+* The FIFO has one byte less capacity than the ``data`` buffer length.
+
+Packets
+-------
+
+Packets are sent over the FIFO described in the above section.
+One packet can be wrapped around if it occurs at the end of the FIFO buffer.
+
+The following is the packet structure:
+
+.. list-table::
+   :header-rows: 1
+
+   * - Field name
+     - Size (bytes)
+     - Byte order
+     - Description
+   * - ``len``
+     - 2
+     - big‑endian
+     - Length of the ``data`` field.
+   * - ``reserved``
+     - 2
+     - n/a
+     - Reserved for the future use.
+       It must be 0 for the current protocol version.
+   * - ``data``
+     - ``len``
+     - n/a
+     - Packet data.
+   * - ``padding``
+     - 0‑3
+     - n/a
+     - Padding is added to align the total packet size to 4 bytes.
+
+The packet send procedure is the following:
+
+#. Check if the packet fits into the buffer.
+#. Write the packet to ``data`` FIFO buffer starting at ``wr_idx``.
+   Wrap it if needed.
+#. Write a new value of the ``wr_idx``.
+#. Notify the receiver over the MBOX channel.
+
+Initialization
+--------------
+
+The initialization sequence is the following:
+
+#. Set the ``wr_idx`` and ``rd_idx`` to zero.
+#. Push a single packet to FIFO containing magic data: ``45 6d 31 6c 31 4b 30 72 6e 33 6c 69 34``.
+   The MBOX is not used yet.
+#. Initialize the MBOX.
+#. Repeat the notification over the MBOX channel using some interval, for example, 1 ms.
+#. Wait for an incoming packet containing the magic data.
+   It will arrive over the other pair (shared memory region and MBOX).
+#. Stop repeating the MBOX notification.
+
+After this, the ICMsg is bound, and it is ready to transfer packets.