doc: add bits about IPC message service

This adds documentation for the newly introduced IPC message
service APIs.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

Author: dcpleung

doc/services/ipc/index.rst

@@ -7,3 +7,4 @@ Interprocessor Communication (IPC)
    :maxdepth: 1
 
    ipc_service/ipc_service.rst
+   ipc_msg_service/ipc_msg_service.rst

doc/services/ipc/ipc_msg_service/ipc_msg_service.rst

@@ -0,0 +1,253 @@
+.. _ipc_msg_service:
+
+IPC Message Service
+###################
+
+.. contents::
+    :local:
+    :depth: 2
+
+The IPC message service API provides an interface to exchange data between two
+domains or CPUs.
+
+Overview
+========
+
+A communication channel consists of one instance and one or several endpoints
+associated with the instance.
+
+An instance is the external representation of a physical communication channel
+between two domains or CPUs. The actual implementation and internal
+representation of the instance is peculiar to each backend.
+
+An individual instance is not used to send data between domains/CPUs. To send
+and receive the data, the user must create (register) an endpoint in the
+instance. This allows for the connection of the two domains of interest.
+
+It is possible to have zero or multiple endpoints for one single instance,
+possibly with different priorities, and to use each to exchange data.
+Endpoint prioritization and multi-instance ability highly depend on the backend
+used and its implementation.
+
+The endpoint is an entity the user must use to send and receive data between
+two domains (connected by the instance). An endpoint is always associated to an
+instance.
+
+The creation of the instances is left to the backend, usually at init time.
+The registration of the endpoints is left to the user, usually at run time.
+
+The API does not mandate a way for the backend to create instances but it is
+strongly recommended to use the devicetree to retrieve the configuration
+parameters for an instance. Currently, each backend defines its own
+DT-compatible configuration that is used to configure the interface at boot
+time.
+
+Simple data exchange
+====================
+
+To send data between domains or CPUs, an endpoint must be registered onto
+an instance.
+
+See the following example:
+
+.. note::
+
+   Before registering an endpoint, the instance must be opened using the
+   :c:func:`ipc_msg_service_open_instance` function.
+
+
+.. code-block:: c
+
+   #include <zephyr/ipc/ipc_msg_service.h>
+
+   static void bound_cb(void *priv)
+   {
+      /* Endpoint bounded */
+   }
+
+   static int recv_cb(uint16_t msg_type, const void *msg_data, void *priv)
+   {
+      /* Data received */
+
+      return 0;
+   }
+
+   static struct ipc_msg_ept_cfg ept0_cfg = {
+      .name = "ept0",
+      .cb = {
+         .bound    = bound_cb,
+         .received = recv_cb,
+      },
+   };
+
+   int main(void)
+   {
+      const struct device *inst0;
+      struct ipc_ept ept0;
+      struct ipc_msg_type_cmd message;
+      int ret;
+
+      inst0 = DEVICE_DT_GET(DT_NODELABEL(ipc0));
+      ret = ipc_msg_service_open_instance(inst0);
+      ret = ipc_msg_service_register_endpoint(inst0, &ept0, &ept0_cfg);
+
+      /* Wait for endpoint bound (bound_cb called) */
+
+      message.cmd = 0x01;
+      ret = ipc_msg_service_send(&ept0, IPC_MSG_TYPE_CMD, &message);
+   }
+
+Querying status
+===============
+
+The API provides a way to perform query on the backend regarding instance
+and endpoint.
+
+See the following example for querying if the endpoint is ready for
+data exchange:
+
+.. code-block:: c
+
+   #include <zephyr/ipc/ipc_msg_service.h>
+
+   static void bound_cb(void *priv)
+   {
+      /* Endpoint bounded */
+   }
+
+   static int recv_cb(uint16_t msg_type, const void *msg_data, void *priv)
+   {
+      /* Data received */
+
+      return 0;
+   }
+
+   static struct ipc_msg_ept_cfg ept0_cfg = {
+      .name = "ept0",
+      .cb = {
+         .bound    = bound_cb,
+         .received = recv_cb,
+      },
+   };
+
+   int main(void)
+   {
+      const struct device *inst0;
+      struct ipc_ept ept0;
+      struct ipc_msg_type_cmd message;
+      int ret;
+
+      inst0 = DEVICE_DT_GET(DT_NODELABEL(ipc0));
+      ret = ipc_msg_service_open_instance(inst0);
+      ret = ipc_msg_service_register_endpoint(inst0, &ept0, &ept0_cfg);
+
+      /* Wait for endpoint bound (bound_cb called) */
+
+      /* Check if endpoint is ready. */
+      ret = ipc_msg_service_query(&ept0, IPC_MSG_QUERY_IS_READY, NULL, NULL);
+      if (ret != 0) {
+         /* Endpoint is not ready */
+      }
+
+      message.cmd = 0x01;
+      ret = ipc_msg_service_send(&ept0, IPC_MSG_TYPE_CMD, &message);
+   }
+
+Events
+======
+
+The backend can also do a callback when certain events come in through
+the instance or endpoint.
+
+See the following example on adding an event callback:
+
+.. code-block:: c
+
+   #include <zephyr/ipc/ipc_msg_service.h>
+
+   static void bound_cb(void *priv)
+   {
+      /* Endpoint bounded */
+   }
+
+   static int recv_cb(uint16_t msg_type, const void *msg_data, void *priv)
+   {
+      /* Data received */
+
+      return 0;
+   }
+
+   static int evt_cb(uint16_t evt_type, const void *evt_data, void *priv)
+   {
+      /* Event received */
+
+      return 0;
+   }
+
+   static struct ipc_msg_ept_cfg ept0_cfg = {
+      .name = "ept0",
+      .cb = {
+         .bound    = bound_cb,
+         .event    = evt_cb,
+         .received = recv_cb,
+      },
+   };
+
+   int main(void)
+   {
+      const struct device *inst0;
+      struct ipc_ept ept0;
+      struct ipc_msg_type_cmd message;
+      int ret;
+
+      inst0 = DEVICE_DT_GET(DT_NODELABEL(ipc0));
+      ret = ipc_msg_service_open_instance(inst0);
+      ret = ipc_msg_service_register_endpoint(inst0, &ept0, &ept0_cfg);
+
+      /* Wait for endpoint bound (bound_cb called) */
+
+      /* Check if endpoint is ready. */
+      ret = ipc_msg_service_query(&ept0, IPC_MSG_QUERY_IS_READY, NULL, NULL);
+      if (ret != 0) {
+         /* Endpoint is not ready */
+      }
+
+      message.cmd = 0x01;
+      ret = ipc_msg_service_send(&ept0, IPC_MSG_TYPE_CMD, &message);
+   }
+
+Backends
+========
+
+The requirements needed for implementing backends give flexibility to the IPC
+message service. These allow for the addition of dedicated backends having only
+a subsets of features for specific use cases.
+
+The backend must support at least the following:
+
+* The init-time creation of instances.
+* The run-time registration of an endpoint in an instance.
+
+Additionally, the backend can also support the following:
+
+* The run-time deregistration of an endpoint from the instance.
+* The run-time closing of an instance.
+* The run-time querying of an endpoint or instance status.
+
+Each backend can have its own limitations and features that make the backend
+unique and dedicated to a specific use case. The IPC message service API can be
+used with multiple backends simultaneously, combining the pros and cons of each
+backend.
+
+API Reference
+=============
+
+IPC Message Service API
+***********************
+
+.. doxygengroup:: ipc_msg_service_api
+
+IPC Message Service Backend API
+*******************************
+
+.. doxygengroup:: ipc_msg_service_backend