Changing STM32MP157D Linux system CLOCK_REALTIME to use the 64-bit Ethernet precision timer

[Mike Thompson]

On an STM32MP157D-based system, I wish to condition the Linux CLOCK_REALTIME precision clock against an external clock source in a non-standard manner (ie. not using PTPD or similar).

To do this, I am using a wireless radio controlled by the CM4 core on the STM32MP157D to synchronize and condition the 64-bit Ethernet precision timer within a microsecond of a clock on the far end of the wireless connection. This code was adapted from an STM32F767 device, and the port was relatively easy to do.

Can I configure the Linux system to use the Ethernet precision timer rather than the default system timer for the CLOCK_REALTIME precision clock? If so, how might I configure this within the kernel or device tree files?

Alternatively, how might I adjust the CM4 ARM Cortex-A Generic Timer, which is likely used by default for CLOCK_REALTIME, so that it can be set and adjusted by code running within the CM4 core? Or is some other clock used by Linux for the CLOCK_REALTIME clock?

Thanks,
Mike

[Mike Thompson]

By the way, I can read the synchronized Ethernet precision timer from a Linux application using the /dev/ptp0 device using the code below. However, it would be best for my purposes if I could get the Linux system clock to use this timer for the CLOCK_REALTIME clock.

#include <time.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <linux/ptp_clock.h>
#include <sys/ioctl.h>

#define FD_TO_CLOCKID(fd)  ((~(unsigned int)(fd) << 3) | 3)

int main() {
    // Open the PHC device (e.g., /dev/ptp0)
    int fd = open("/dev/ptp0", O_RDONLY);
    if (fd < 0) {
        perror("Failed to open PHC device");
        return -1;
    }

    // Convert file descriptor to clock ID
    clockid_t clk_id = FD_TO_CLOCKID(fd);

    // Use clock_gettime to fetch the current time from the PHC
    struct timespec ts;
    if (clock_gettime(clk_id, &ts) < 0) {
        perror("Failed to get PHC time");
        close(fd);
        return -1;
    }

    printf("PHC time: %lld.%09ld seconds\n", (long long)ts.tv_sec, ts.tv_nsec);

    // Close the PHC device
    close(fd);
    return 0;
}