Tiny helper to set ASYNC_LOW_LATENCY on Linux Serial Ports, since setserial is dead.

lowlatency.c

// Tiny helper to set ASYNC_LOW_LATENCY on Linux Serial Ports, since setserial is dead.
// ISC (c) 2025 Adrian "vifino" Pistol

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <getopt.h>
#include <sys/ioctl.h>
#include <linux/serial.h>

static void usage(const char *prog) {
	fprintf(stderr,
		"Usage: %s [ -o | --on ] [ -f | --off ] <device>\n"
		"\t-e, --enable\t\tenable ASYNC_LOW_LATENCY\n"
		"\t-f, --disable\t\tdisable ASYNC_LOW_LATENCY\n",
		prog);
	exit(1);
}

int main(int argc, char *argv[]) {
	int enable = -1;
	int c;

	static struct option long_opts[] = {
		{"enable",  no_argument, 0, 'e'},
		{"disable", no_argument, 0, 'd'},
		{0,         0,           0,  0 }
	};

	while ((c = getopt_long(argc, argv, "ed", long_opts, NULL)) != -1) {
		switch (c) {
		case 'e':
			enable = 1;
			break;
		case 'd':
			enable = 0;
			break;
		default:
			usage(argv[0]);
		}
	}

	if (enable < 0 || optind >= argc) {
		usage(argv[0]);
	}

	const char *dev = argv[optind];
	int fd = open(dev, O_RDONLY | O_NONBLOCK);
	if (fd < 0) {
		perror("open");
		return 1;
	}

	struct serial_struct ser;
	if (ioctl(fd, TIOCGSERIAL, &ser) < 0) {
		perror("TIOCGSERIAL");
		close(fd);
		return 1;
	}

	if (enable)
		ser.flags |= ASYNC_LOW_LATENCY;
	else
		ser.flags &= ~ASYNC_LOW_LATENCY;

	if (ioctl(fd, TIOCSSERIAL, &ser) < 0) {
		perror("TIOCSSERIAL");
		close(fd);
		return 1;
	}

	close(fd);
	return 0;
}