[nrf fromlist] ipc: fix return code of icbmsg backend send operation.

Upstream PR: zephyrproject-rtos/zephyr#73156

This commit fixes the issue where a serialization
error was reported after properly sending a data with 'icbmsg' backend.

The icbmsg send function's return code is set to
the sent data's len as in other backends.
The related docs were fixed and updated.

Signed-off-by: Piotr Koziar <piotr.koziar@nordicsemi.no>

Author: piotrkoziar

include/zephyr/ipc/icmsg.h

@@ -111,7 +111,7 @@ int icmsg_close(const struct icmsg_config_t *conf,
  *  @param[in] len Size of data in the @p msg buffer.
  *
  *
- *  @retval 0 on success.
+ *  @retval Number of sent bytes.
  *  @retval -EBUSY when the instance has not finished handshake with the remote
  *                 instance.
  *  @retval -ENODATA when the requested data to send is empty.

subsys/ipc/ipc_service/backends/ipc_icbmsg.c

@@ -501,7 +501,7 @@ static int send_control_message(struct backend_data *dev_data, enum msg_type msg
 	r = icmsg_send(&conf->control_config, &dev_data->control_data, &message,
 		       sizeof(message));
 	k_mutex_unlock(&dev_data->mutex);
-	if (r < 0) {
+	if (r < sizeof(message)) {
 		LOG_ERR("Cannot send over ICMsg, err %d", r);
 	}
 	return r;
@@ -541,7 +541,7 @@ static int send_release(struct backend_data *dev_data, const uint8_t *buffer,
  * @param[in] size		Actual size of the data, can be smaller than allocated,
  *				but it cannot change number of required blocks.
  *
- * @return			O or negative error code.
+ * @return			number of bytes sent in the message or negative error code.
  */
 static int send_block(struct backend_data *dev_data, enum msg_type msg_type,
 		      uint8_t ept_addr, size_t tx_block_index, size_t size)
@@ -656,7 +656,7 @@ static int match_bound_msg(struct backend_data *dev_data, size_t rx_block_index,
  *
  * @param[in] ept	Endpoint to use.
  *
- * @return		O or negative error code.
+ * @return		non-negative value in case of success or negative error code.
  */
 static int send_bound_message(struct backend_data *dev_data, struct ept_data *ept)
 {
@@ -992,7 +992,12 @@ static int send(const struct device *instance, void *token, const void *msg, siz
 	memcpy(buffer, msg, len);
 
 	/* Send data message. */
-	return send_block(dev_data, MSG_DATA, ept->addr, r, len);
+	r = send_block(dev_data, MSG_DATA, ept->addr, r, len);
+	if (r < 0) {
+		return r;
+	}
+
+	return len;
 }
 
 /**