Setting /dev/ttyS0 baud rate to >115200

Question

I am trying to set the baud rate of the serial port (/dev/ttyS0) to 1Mb (1000000). I've tried writing code using cfsset{i|o}speed() + tc{g|s}etattr(). Anything >115200 is ignored.

#include <termio.h>
#include <fcntl.h>
#include <err.h>
#include <linux/serial.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

static int rate_to_constant(int baudrate) 
{
#define B(x) case x: return B##x
  switch(baudrate) {
    B(50);     B(75);     B(110);    B(134);    B(150);
    B(200);    B(300);    B(600);    B(1200);   B(1800);
    B(2400);   B(4800);   B(9600);   B(19200);  B(38400);
    B(57600);  B(115200); B(230400); B(460800); B(500000); 
    B(576000); B(921600); B(1000000);B(1152000);B(1500000);

  default: return 0;
  }
#undef B
}    

/* Open serial port in raw mode, with custom baudrate if necessary */
int serial_open(const char *device, int rate)
{
  struct termios options;
  struct termios options1;
  struct serial_struct serial;
  int fd;
  int speed = 0;

  /* Open and configure serial port */
  if ((fd = open(device, O_RDWR | O_NOCTTY)) == -1)
    return -1;

  speed = rate_to_constant(rate);
  warnx("speed: %d", speed);

  fcntl(fd, F_SETFL, 0);

  if (speed == 0)
    {
      warnx("speed == 0");

      /* Custom divisor */
      memset(&serial, 0, sizeof(struct serial_struct));

      serial.reserved_char[0] = 0;

      if (ioctl(fd, TIOCGSERIAL, &serial) < 0) return -1;

      warnx("serial.baud_base: %d", serial.baud_base);

            serial.flags &= 
      serial.flags &= ~ASYNC_SPD_MASK;
      serial.flags |= ASYNC_SPD_CUST;

      serial.custom_divisor = (serial.baud_base + (rate / 2)) / rate;

      warnx("serial.custom_divisor: %d", serial.custom_divisor);

      if (serial.custom_divisor < 1)
    serial.custom_divisor = 1;

      if (ioctl(fd, TIOCSSERIAL, &serial) < 0)
    return -1;

      if (ioctl(fd, TIOCGSERIAL, &serial) < 0)
    return -1;

      if (serial.custom_divisor * rate != serial.baud_base)
        {
          warnx("actual baudrate is %d / %d = %f",
                serial.baud_base, serial.custom_divisor,
                (float)(serial.baud_base) / serial.custom_divisor);
        }
    }

  memset(&options, 0, sizeof(struct termios));
  tcgetattr(fd, &options);
  cfmakeraw(&options);

  cfsetispeed(&options, speed ?: B38400);
  cfsetospeed(&options, speed ?: B38400);

  options.c_cflag |= (CLOCAL | CREAD);
  options.c_cflag &= ~CRTSCTS;

  if (tcsetattr(fd, TCSANOW, &options) != 0)
    return -1;

  memset(&options1, 0, sizeof(struct termios));
  tcgetattr(fd, &options1);
  warn("options1.speeds: %d and %d", cfgetispeed(&options1), cfgetospeed(&options1));

  return fd;
}

int main(int argc, char *argv[])
{
  int fd;

  //fd = serial_open("/dev/ttyS0", 115200);
  fd = serial_open("/dev/ttyS0", 460800);

  warnx("END");

  close(fd);
}

This is based on: How to set baud rate to 307200 on Linux?

I've tried using stty and setserial on the command line ... same results.

stty -F /dev/ttyS0 1000000

1) Do I need to change kernel code (serial_core.c or 2850.c or 2850_pci.c or etc.)?

2) Is there user-space code that will do?

Elżbieta

Have you verified in first place that your serial port can support this speed? — Alexandre Lavoie, Jan 07 '16 at 22:37

score 1 · Answer 1 · answered Apr 29 '20 at 09:22

If the bit rate you want to set is defined in <asm/termbits.h> (as included from <termios.h>), then it's cake, and there's no need to go through all that custom divisor mish-mash (which is device-specific, anyway):

struct termios ttycfg;
int            fd;

fd = open ("/dev/ttyS0", O_RDWR | O_NOCTTY);
tcgetattr (fd, &ttycfg);
cfsetspeed (&ttycfg, B961200);
tcsetattr (fd, TCSANOW, &ttycfg);

Note that tcsetattr() will return success if any of the requested changes could be carried out, so if you're changing multiple settings, you should follow up with another tcgetattr() to see which ones were successfully applied.

Aside: Many Debian-based systems have a package called modemmanager installed by default, which purports to detect and manage modems by opening every serial port it can find, shouting AT commands down the wire, and examining the responses (JTAG adapters just love having this done to them :-/ ). If you're dealing with hotpluggable serial ports (such as with USB serial adapters), this thing will ruin your day, so you'll want to configure it to keep it away from the ports you're using (or just delete it entirely).

Setting /dev/ttyS0 baud rate to >115200

1 Answers1