Harnessing the Power of Remote Procedure Calls on the i.MX8: A Comprehensive Guide for Firmware Development

The i.MX8 System on Chip (SoC) represents a significant leap in embedded processing capabilities, offering a potent combination of heterogeneous cores designed to tackle complex tasks concurrently. At revWhiteShadow, we understand the intricate challenges faced by developers when orchestrating the interplay between these diverse processing units. Specifically, a common and critical requirement is the seamless communication and control of the Cortex-M4 core residing within the i.MX8 SoC, often managed by a higher-level Linux environment running on the Cortex-A53 cores. This article delves deeply into the practical application and optimization of remote procedure calls (RPC) as the definitive mechanism for achieving this sophisticated inter-core communication. We will explore the underlying principles, essential setup procedures, and advanced techniques that enable robust and efficient firmware development for the i.MX8 platform. Our focus will be on providing actionable insights and detailed guidance, drawing from our extensive experience in embedded systems and firmware engineering.

Understanding the i.MX8 Architecture and Inter-Core Communication Needs

The NXP i.MX8 family, particularly variants like the MIMX8M5, is characterized by its multicore architecture. This typically includes one or more powerful Cortex-A application processors, designed to run rich operating systems like Linux, and one or more smaller, more power-efficient Cortex-M microcontrollers. The Cortex-M4 core, in this context, is often relegated to real-time tasks, hardware control, sensor management, or low-level peripheral operations that demand deterministic execution and minimal latency.

The challenge, as highlighted in many developer discussions, arises from the need for the Linux environment on the Cortex-A53 to initiate, monitor, and receive data from tasks executing on the Cortex-M4. This necessitates a well-defined communication channel that transcends the boundaries of individual cores and operating system contexts. Simply put, the Cortex-A53 needs a way to tell the Cortex-M4 what to do and to get results back, reliably and efficiently.

Traditional methods might involve shared memory with complex synchronization primitives, interrupt-driven messaging, or dedicated hardware FIFOs. However, these approaches can quickly become unwieldy, difficult to debug, and prone to race conditions or performance bottlenecks. This is precisely where the elegance and robustness of remote procedure calls shine. RPC allows the Cortex-A53 to invoke functions residing on the Cortex-M4 as if they were local calls, abstracting away the complexities of the underlying communication.

The Case for Remote Procedure Calls on the i.MX8

Remote Procedure Calls (RPC) offer a powerful abstraction layer for inter-process and inter-core communication. In the context of the i.MX8, an RPC framework enables the Linux application running on the Cortex-A53 to invoke specific functions or methods implemented within the Cortex-M4 firmware. The underlying RPC mechanism handles the serialization of arguments, transmission across the communication channel, execution of the remote function, and the deserialization and return of results.

Why is RPC particularly well-suited for the i.MX8 MIMX8M5 scenario?

Abstraction and Simplicity: Developers on the Cortex-A53 can interact with the Cortex-M4 without needing to understand the low-level details of the communication protocol, memory mapping, or interrupt handling. This significantly simplifies application development and reduces the cognitive load on engineers.
Modularity and Reusability: RPC promotes a modular design. Firmware components on the Cortex-M4 can be treated as independent services that can be called upon as needed. This encourages code reuse and makes it easier to update or replace individual modules without affecting the entire system.
Efficiency for Task-Oriented Communication: For scenarios where the Cortex-A53 needs to command the Cortex-M4 to perform a specific task (e.g., “read sensor data,” “configure peripheral,” “start motor”), RPC provides a natural and efficient paradigm. The overhead is justified by the clear invocation and response structure.
Robust Error Handling: Well-designed RPC frameworks incorporate mechanisms for handling communication errors, timeouts, and exceptions, contributing to a more resilient system.

Establishing the Communication Channel: rpmsg and the Device Tree

The primary mechanism for enabling RPC between Linux and the Cortex-M cores on NXP i.MX SoCs is rpmsg, a VirtIO-based inter-processor communication (IPC) mechanism. rpmsg is designed to provide a reliable, efficient, and standardized way for different processing environments within an SoC to exchange messages.

rpmsg operates over a shared memory buffer, managed by the system’s memory management unit. For rpmsg to function correctly, the hardware resources, specifically the shared memory regions and control mechanisms, must be explicitly defined and allocated. This is where the device tree plays a crucial role.

The Device Tree: The Cornerstone of i.MX8 Resource Management

The device tree is a data structure that describes the hardware of a system. In the context of embedded Linux, it’s used to provide information about peripherals, memory regions, interrupt controllers, and other hardware components to the kernel. For inter-processor communication like rpmsg, the device tree is essential for:

Defining Shared Memory Regions: The device tree specifies the physical memory addresses and sizes of the shared buffers that will be used by rpmsg. This ensures that both the Cortex-A53 (Linux) and the Cortex-M4 firmware have a clear understanding of where to access and write data.
Specifying rpmsg Endpoints: rpmsg uses the concept of “endpoints” to identify communication channels. The device tree declares these endpoints, associating them with specific memory regions and providing identifiers that allow the two cores to find and connect to each other.
Configuring Resource Allocation: The device tree can also be used to hint at or directly manage resources like DMA channels, interrupts, or clocks that might be necessary for rpmsg operation, although rpmsg primarily relies on shared memory.

Key Device Tree Nodes for rpmsg

When configuring the device tree for rpmsg on an i.MX8, you will typically encounter nodes related to the Interconnect Matrix (i.MX8’s internal bus fabric) and specific nodes that describe the shared memory configurations for rpmsg.

A typical entry might look something like this within the device tree source (.dts file):

/* Example Snippet within your i.MX8 DTS */

/ {
    reserved-memory {
        #address-cells = <2>;
        #size-cells = <2>;
        ranges;

        /* Define a shared memory region for rpmsg */
        /* This region needs to be accessible by both cores */
        rpmsg_shm: rpmsg_shm@<address> {
            compatible = "shared-dma-pool";
            reg = <<<address> <size>>;
            no-map; /* For simplicity, we let the kernel map it */
        };
    };

    firmware {
        /* This node might describe the Cortex-M4 firmware loading */
        /* and its association with rpmsg */
        imx8m5_cm4_firmware: cortexm4@<address> {
            compatible = "fsl,imx8m5-firmware"; /* Custom compatible string */
            reg = <<<address> <size>>; /* Memory where the M4 firmware resides */
            fsl,firmware = "cm4_firmware.bin"; /* Path to the firmware binary */

            /* Crucially, link the rpmsg endpoints */
            mboxes = <&rpmsg_shm>; /* Pointer to the shared memory region */
            /* Add any other necessary properties like interrupts */
        };
    };
};

/* rpmsg driver node */
/* This node might be under a specific bus controller or general device node */
rpmsg: rpmsg@<address> { /* Base address of the rpmsg controller if applicable */
    compatible = "fsl,imx8-rpmsg"; /* Or the relevant rpmsg compatible string */
    #address-cells = <1>;
    #size-cells = <0>;
    ranges;

    /* Define the endpoints for communication */
    /* The 'name' property is often used for identification */
    /* The 'id' property is the source/destination endpoint identifier */
    /* The 'fsl,shm-offset' property points into the shared memory region */
    /* The 'property' for the M4 side will often be defined in its own context */

    /* Example for Linux (console) endpoint */
    console: console@1 { /* Endpoint ID 1 for console */
        reg = <1>;
        compatible = "rpmsg-tty";
        name = "VirtualConsole";
        fsl,shm-offset = <0x0000>; /* Offset within rpmsg_shm */
        fsl,shm-size = <0x1000>;   /* Size of this particular channel */
    };

    /* Example for a custom application endpoint */
    my_app_service: my_app_service@2 { /* Endpoint ID 2 for custom app */
        reg = <2>;
        compatible = "rpmsg-channel"; /* Generic rpmsg channel */
        name = "MyAppService";
        fsl,shm-offset = <0x1000>; /* Offset within rpmsg_shm */
        fsl,shm-size = <0x8000>;   /* Size of this particular channel */
    };

    /* ... potentially other endpoints ... */
};

Important Considerations for Device Tree:

Addressing: Ensure the reg properties for shared memory and firmware correctly reflect the physical memory map of your i.MX8 board.
Compatibility Strings: The compatible strings are critical for the Linux kernel and the Cortex-M firmware (if it uses an rpmsg framework that parses the device tree) to identify and bind to the correct drivers. You might need to define custom compatible strings if using a highly specialized framework.
Endpoint IDs: Endpoint IDs are crucial. Linux typically uses a master ID (e.g., 0x0) for control messages and then assigns unique IDs for other services. The Cortex-M firmware must also adhere to these IDs or use a negotiation mechanism.
Shared Memory Management: The rpmsg_shm node’s reg property defines the base address and size of the memory pool. The fsl,shm-offset and fsl,shm-size within endpoint nodes specify how this pool is partitioned for individual communication channels.

Cortex-M Firmware Setup for rpmsg

On the Cortex-M4 side, you will need an rpmsg framework or library that can interface with the shared memory and manage the communication protocol. NXP often provides its own Software Development Kit (SDK) or examples that include an rpmsg implementation for their Cortex-M cores. This framework will typically:

Initialize Shared Memory: Access the shared memory regions as defined by the device tree (or configured directly if not using device tree binding for the M4).
Register Endpoints: Create and register endpoints with their corresponding IDs.
Handle Incoming Messages: Listen for incoming messages on registered endpoints.
Send Outgoing Messages: Transmit messages to specified endpoints.
Manage State: Maintain the state of the communication channels.

A common approach is to have the Cortex-M firmware expose a set of RPC functions. When a message arrives from Linux that represents an RPC request, the rpmsg framework on the M4 will parse the message, identify the requested function, call the corresponding C function in the firmware, serialize the return value, and send it back to Linux.

Implementing RPC Functions on the Cortex-M4

The core of your RPC implementation on the Cortex-M4 will involve defining functions that can be called remotely. These functions will typically perform specific tasks, such as reading sensor data, controlling a GPIO pin, or performing a computation.

Let’s consider a scenario where the Cortex-A53 needs to request the current temperature reading from a sensor managed by the Cortex-M4.

Defining the RPC Interface (Cortex-M4 Side)

You would define a C function in your Cortex-M4 firmware that performs the actual sensor reading. This function should be designed to accept necessary parameters and return the sensor value.

// In your Cortex-M4 firmware (e.g., sensor_driver.c)

#include "rpmsg_lite.h" // Assuming an rpmsg_lite library is used
#include "sensor_api.h" // Your sensor interaction library

// Define a unique identifier for this RPC service
#define MY_APP_SERVICE_ENDPOINT_ID 2 // Must match device tree
#define GET_TEMPERATURE_RPC_ID 1     // Identifier for the get_temperature RPC

// Structure to hold RPC arguments and results
typedef struct {
    int rpc_id;
    union {
        // Arguments for GET_TEMPERATURE
        struct {
            // No arguments needed for this example
        } get_temp_args;
        // ... other RPC argument structures
    } args;
    union {
        // Results for GET_TEMPERATURE
        struct {
            float temperature;
        } get_temp_result;
        // ... other RPC result structures
    } results;
} rpc_message_t;

// Global handle for the rpmsg endpoint
static struct rpmsg_lite_instance *my_app_rpmsg_instance;

// Function to handle incoming RPC requests
static int handle_rpc_request(struct rpmsg_lite_instance *rpmsg_inst,
                               struct rpmsg_lite_mbuf *msg,
                               void *priv)
{
    rpc_message_t *rpc_msg = (rpc_message_t *)msg->data;
    int ret_status = 0; // Success code

    switch (rpc_msg->rpc_id) {
        case GET_TEMPERATURE_RPC_ID:
            {
                // Call the actual sensor reading function
                float temp = sensor_read_temperature();

                // Populate the result
                rpc_msg->results.get_temp_result.temperature = temp;
                ret_status = 0; // Indicate success
            }
            break;
        // ... handle other RPC IDs ...
        default:
            ret_status = -1; // Unknown RPC ID
            break;
    }

    // Prepare the response message (including status)
    rpc_msg->rpc_id = rpc_msg->rpc_id | 0x80000000; // Indicate response, use high bit
    // Set return status if your framework supports it, or embed in data
    // For simplicity, we'll just send back the populated result

    // Send the response back to the sender
    rpmsg_lite_send(rpmsg_inst, msg, (char *)rpc_msg, sizeof(rpc_message_t));

    return RPMSG_LITE_SUCCESS;
}

// Initialization function for the RPC service
void my_app_rpc_init(void)
{
    // Initialize rpmsg_lite and create/bind the endpoint
    // This would typically involve getting the rpmsg_lite_instance
    // from the system's rpmsg driver and creating a named endpoint.
    // Example:
    // my_app_rpmsg_instance = rpmsg_lite_open(rpmsg_shm_base, rpmsg_shm_size, "MyAppService", RPMSG_LITE_PROBE);
    // rpmsg_lite_register_callback(my_app_rpmsg_instance, handle_rpc_request, NULL);

    // For simplicity, assume my_app_rpmsg_instance is already initialized
    // and registered with the handle_rpc_request callback
}

Structuring RPC Messages

A robust RPC system needs a well-defined message format. We’ve used a simple rpc_message_t struct in the example. This struct should contain:

An RPC identifier: A unique number that specifies which function to call.
Arguments: Data passed to the remote function. This can be a union of different argument structures for different RPCs.
Return Values: Data returned by the remote function. Again, a union is suitable.
Status/Error Codes: To indicate success or failure of the remote execution.

Serialization: When passing complex data types (structs, arrays) or non-native types, you’ll need to serialize them into a byte stream for transmission over rpmsg and deserialize them on the receiving end. Libraries like cbor, protobuf, or even custom simple serialization routines can be employed. For basic data types like float or int, direct embedding in the message struct is often sufficient if alignment and endianness are handled correctly.

Initiating RPC Calls (Cortex-A53 Side - Linux)

On the Linux side, you’ll use the rpmsg kernel driver and potentially a user-space library to interact with the rpmsg endpoints. The Linux kernel exposes rpmsg devices through the character device interface (e.g., /dev/rpmsg_char_devX).

A user-space application would typically:

Open the rpmsg device: open("/dev/rpmsgX", ...)
Create/Bind to an endpoint: Use ioctl calls or specific library functions to create a local endpoint and advertise a service name (e.g., “MyAppService”) to match the Cortex-M service.
Send an RPC Request: Construct the rpc_message_t struct for the desired RPC (e.g., GET_TEMPERATURE_RPC_ID) and send it using write() or sendmsg() to the rpmsg device.
Receive the RPC Response: Use read() or recvmsg() to wait for and receive the reply from the Cortex-M4.
Parse the Response: Deserialize the received data into the rpc_message_t structure and extract the results.

Example User-Space Code Snippet (Conceptual)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <rpmsg_lite.h> // Assuming a user-space rpmsg library or equivalent interface

// Define RPC IDs matching the M4 side
#define GET_TEMPERATURE_RPC_ID 1

// Message structure mirroring the M4 side
typedef struct {
    int rpc_id;
    union {
        struct { /* Args for GET_TEMPERATURE */ } get_temp_args;
        // ...
    } args;
    union {
        struct { float temperature; } get_temp_result;
        // ...
    } results;
} rpc_message_t;

int main() {
    int fd;
    rpc_message_t request_msg;
    rpc_message_t response_msg;

    // Find the correct rpmsg device node (this can be complex in practice)
    // For simplicity, let's assume it's /dev/rpmsg0
    fd = open("/dev/rpmsg0", O_RDWR);
    if (fd < 0) {
        perror("Failed to open rpmsg device");
        return -1;
    }

    // --- Binding to the service ---
    // This would typically involve an ioctl to bind to the "MyAppService"
    // and potentially negotiate endpoints. The actual mechanism depends
    // on the specific rpmsg user-space library or driver interface.
    // For instance, you might need to:
    // 1. Create a local endpoint ID.
    // 2. Send a bind request to the M4 for "MyAppService".
    // 3. Receive a bind acknowledgement and the M4's endpoint ID.

    // For this conceptual example, we assume we know the M4's endpoint (e.g., endpoint ID 2)
    // and we have a mechanism to send messages directly to it.

    // --- Sending the RPC Request ---
    request_msg.rpc_id = GET_TEMPERATURE_RPC_ID;
    // No arguments for this RPC, so args union is empty.

    // Send the message to the M4's endpoint (e.g., endpoint ID 2)
    // The send mechanism depends on the user-space rpmsg interface.
    // Assuming a simple write call to the rpmsg device:
    if (write(fd, &request_msg, sizeof(request_msg)) < 0) {
        perror("Failed to send RPC request");
        close(fd);
        return -1;
    }

    // --- Receiving the RPC Response ---
    // Wait for a response. This could be a blocking read.
    if (read(fd, &response_msg, sizeof(response_msg)) < 0) {
        perror("Failed to receive RPC response");
        close(fd);
        return -1;
    }

    // --- Processing the Response ---
    if ((response_msg.rpc_id >> 31) && (response_msg.rpc_id & 0x7FFFFFFF) == GET_TEMPERATURE_RPC_ID) {
        printf("Received temperature: %.2f degrees Celsius\n", response_msg.results.get_temp_result.temperature);
    } else {
        printf("Received an unexpected response or error.\n");
    }

    close(fd);
    return 0;
}

Advanced Considerations and Optimization

While the basic RPC mechanism is functional, several factors can significantly improve performance and reliability.

Message Buffering and Pooling

The allocation and deallocation of message buffers can be a performance bottleneck. Implementing a message buffer pool on both the Cortex-M4 and potentially in user-space on Linux can reduce overhead associated with dynamic memory allocation. Pre-allocating a set of message buffers and reusing them minimizes fragmentation and latency.

Serialization Efficiency

For complex data structures or high-frequency updates, the efficiency of your serialization and deserialization routines is paramount. Consider:

Minimal Overhead Formats: Using compact binary formats like CBOR or Protocol Buffers can be more efficient than text-based formats like JSON.
Zero-Copy Mechanisms: If your rpmsg framework supports it, investigate zero-copy techniques where data is directly accessed from the shared memory without intermediate copying.
Optimized C Libraries: For C/C++ code, ensure your serialization functions are well-optimized.

Error Handling and Timeouts

A robust RPC system must gracefully handle failures. This includes:

Timeouts: Implement timeouts on RPC requests to prevent the calling core from blocking indefinitely if the remote core is unresponsive.
Error Codes: Standardize error codes to communicate specific failure reasons (e.g., “function not found,” “invalid arguments,” “resource unavailable”).
Retries: For transient errors, consider implementing a retry mechanism, especially for critical operations.

Concurrency and Thread Safety

On both the Cortex-M4 (if using an RTOS) and Linux (which is multithreaded), ensure your RPC handlers and the underlying rpmsg framework are thread-safe. If multiple threads on the Linux side can initiate RPC calls to the same Cortex-M service, proper synchronization is essential. Similarly, if the Cortex-M firmware uses an RTOS, shared resources accessed by RPC handlers must be protected.

Service Discovery and Registration

For systems with multiple services running on the Cortex-M4, a mechanism for service discovery can be beneficial. The Cortex-M firmware could advertise its services upon initialization, and the Linux application could then discover and connect to them. This adds flexibility and allows for dynamic loading/unloading of services.

Leveraging OpenWrt’s Capabilities

Since you are running OpenWrt on the Cortex-A53, you have access to a rich Linux ecosystem. This means you can:

Develop User-space Libraries: Create dedicated C or C++ libraries that abstract the rpmsg communication and RPC calls, making it easier for other applications on OpenWrt to utilize the Cortex-M functionality.
Integrate with Existing Frameworks: If your OpenWrt build includes tools like dbus or other middleware, you can potentially bridge RPC calls over to these frameworks for broader system integration.
Systemd Services: Encapsulate your Linux-side RPC client logic within a systemd service for robust management, automatic startup, and logging.

Conclusion

The integration of Cortex-M firmware with a Linux environment on the i.MX8 SoC, particularly using rpmsg and remote procedure calls, is a powerful approach for building complex embedded systems. By carefully defining the device tree, implementing robust RPC interfaces on the Cortex-M4, and creating efficient client applications on the Linux side, developers can achieve seamless and reliable inter-core communication. At revWhiteShadow, we advocate for this structured approach to harness the full potential of the i.MX8’s heterogeneous architecture, enabling sophisticated control and data exchange between the application processors and the real-time cores. Mastering these techniques will undoubtedly lead to more performant, maintainable, and scalable embedded firmware solutions.

Using remote-proc on the imx8

Harnessing the Power of Remote Procedure Calls on the i.MX8: A Comprehensive Guide for Firmware Development #

Understanding the i.MX8 Architecture and Inter-Core Communication Needs #

The Case for Remote Procedure Calls on the i.MX8 #

Establishing the Communication Channel: rpmsg and the Device Tree #

The Device Tree: The Cornerstone of i.MX8 Resource Management #

Key Device Tree Nodes for rpmsg #

Cortex-M Firmware Setup for rpmsg #

Implementing RPC Functions on the Cortex-M4 #

Defining the RPC Interface (Cortex-M4 Side) #

Structuring RPC Messages #

Initiating RPC Calls (Cortex-A53 Side - Linux) #

Example User-Space Code Snippet (Conceptual) #

Advanced Considerations and Optimization #

Message Buffering and Pooling #

Serialization Efficiency #

Error Handling and Timeouts #

Concurrency and Thread Safety #

Service Discovery and Registration #

Leveraging OpenWrt’s Capabilities #

Conclusion #