#include <errno.h>
#include <stdarg.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>

__attribute__((noreturn)) void die(const char *fmt, ...)
{
	va_list ap;

	va_start(ap, fmt);
	vfprintf(stderr, fmt, ap);
	va_end(ap);

	fprintf(stderr, "\n\n");

	exit(EXIT_FAILURE);
}


int main (int argc, char **argv) {
	uint32_t *an;
	void *ptr;
	int fd;

	fd = open("/dev/mem", O_RDWR);
	if (fd < 0)
		die("Cannot open /dev/mem: %m");

	ptr = mmap(NULL, 0x1000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0xf1036000);
	if (ptr == MAP_FAILED)
		die("Cannot mmap /dev/mem: %m");

	an = ptr + 0xc0c;
	printf("Port Auto-Negotiation Configuration Register = %#010x\n", *an);
	if (argc > 1) {
		unsigned long new_an;
		char *end;

		new_an = strtoul(argv[1], &end, 16);
		if (*end != '\0' || (new_an == ULONG_MAX && errno == ERANGE))
			die("Invalid value %s", argv[1]);

		printf("Setting value = %#010lx\n", new_an);
		*an = new_an;
	}

	munmap(ptr, 0x1000);
	close(fd);

	return 0;
}