ICMsg backend

The inter core messaging backend (ICMsg) is a lighter alternative to the heavier RPMsg static vrings backend. It offers a minimal feature set in a small memory footprint. The ICMsg backend is build on top of Single Producer Single Consumer Packet Buffer.


The ICMsg backend uses shared memory and MBOX devices for exchanging data. Shared memory is used to store the data, MBOX devices are used to signal that the data has been written.

The backend supports the registration of a single endpoint on a single instance. If the application requires more than one communication channel, you must define multiple instances, each having its own dedicated endpoint.


The backend is configured via devicetree. When configuring the backend, do the following:

  • Define two memory regions and assign them to tx-region and rx-region of an instance. Ensure that the memory regions used for data exchange are unique (not overlapping any other region) and accessible by both domains (or CPUs).

  • Define MBOX devices which are used to send the signal that informs the other domain (or CPU) that data has been written. Ensure that the other domain (or CPU) is able to receive the signal.

See the following configuration example for one of the instances:

reserved-memory {
   tx: memory@20070000 {
      reg = <0x20070000 0x0800>;

   rx: memory@20078000 {
      reg = <0x20078000 0x0800>;

   ipc {
      ipc0: ipc0 {
         compatible = "zephyr,ipc-icmsg";
         tx-region = <&tx>;
         rx-region = <&rx>;
         mboxes = <&mbox 0>, <&mbox 1>;
         mbox-names = "tx", "rx";
         status = "okay";

You must provide a similar configuration for the other side of the communication (domain or CPU) but you must swap the MBOX channels and memory regions (tx-region and rx-region).


When the endpoint is registered, the following happens on each domain (or CPU) connected through the IPC instance:

1. The domain (or CPU) writes a magic number to its tx-region of the shared memory. #. It then sends a signal to the other domain or CPU, informing that the data has been written. #. It then checks, regardless of the signal from the other side being received or not, if the magic number is already in its rx-tegion. If it is, ipc_service_cb.bound is called. #. If the magic number is not yet in the rx-region, the domain (or CPU) waits to receive the signal from the other side of the communication to finish the bonding process, and informs the application by calling ipc_service_cb.bound.

Since the ICMsg backend performs the first read from rx-region, regardless of the signaling mechanism, the shared memory areas must be cleared with zeroes before any side of the communication uses them.