How to detect if a driver not supports the ioctl command TIOCSSERIAL

Question

I am using USB to RS-232 serial adapters and cannot set the line properties to use custom baud rates on linux (fedora 26 or fedora 32) using:

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

#include "portutils.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
}

int main(int argc, char** argv) {

    struct termios options;
    struct serial_struct serinfo;
    int fd;
    int speed = 0;
    int baudrate = 625000;
    char * port_name;

      // Check arguments for correct usage
    if (argc != 3){
        printf("\n  Usage: %s port baudrate!\n\n", argv[0]);
        return -1;
    }

    print_bar();

    baudrate = atoi(argv[2]);
    port_name = argv[1];

    /* Open and configure serial port */
    if ((fd = open(port_name, O_RDWR|O_NOCTTY)) == -1)
    {
        printf("\n  Could not open port %s\n", port_name);
        goto HELL;
    }

    printf("  Trying to set baud rate to %d\n  On port %s\n", baudrate, port_name);

    // if you've entered a standard baud the function below will return it
    speed = rate_to_constant(baudrate);

    if (speed == 0) {
        /* Custom divisor */
        serinfo.reserved_char[0] = 0;
        if (ioctl(fd, TIOCGSERIAL, &serinfo) < 0) {
            printf("\n  ioctl: Failed to get #1\n  serial line information of %s\n", port_name);
            goto HELL;
        }
        serinfo.flags &= ~ASYNC_SPD_MASK;
        serinfo.flags |= ASYNC_SPD_CUST;
        serinfo.custom_divisor = (serinfo.baud_base + (baudrate / 2)) / baudrate;
        if (serinfo.custom_divisor < 1) {
            serinfo.custom_divisor = 1;
        }
        printf("  Baud_base: %d\n", serinfo.baud_base);
        printf("  Devisor: %d\n", serinfo.custom_divisor);
        printf("  Resulting baudrate: %f\n", serinfo.baud_base/(1.0*serinfo.custom_divisor));        
        if (ioctl(fd, TIOCSSERIAL, &serinfo) < 0) {
            printf("\n  ioctl: Failed to set\n  serial line information of %s\n", port_name);
            goto HELL;

        }
        if (ioctl(fd, TIOCGSERIAL, &serinfo) < 0) {
            printf("\n  ioctl: Failed to get #2\n  serial line information of %s\n", port_name);
            goto HELL;
        }
        if (serinfo.custom_divisor * baudrate != serinfo.baud_base) {
            warnx("actual baudrate is %d / %d = %f",
                  serinfo.baud_base, serinfo.custom_divisor,
                  (float)serinfo.baud_base / serinfo.custom_divisor);
        }
    }

    fcntl(fd, F_SETFL, 0);
    tcgetattr(fd, &options);
    cfsetispeed(&options, speed ?: B38400);
    cfsetospeed(&options, speed ?: B38400);
    cfmakeraw(&options);
    options.c_cflag |= (CLOCAL | CREAD);
    options.c_cflag &= ~CRTSCTS;
    if (tcsetattr(fd, TCSANOW, &options) != 0)
    {
        printf("\n  Failed to set final attributes of %s\n\n", port_name);
        goto HELL;
    }

    close(fd);
    printf("  Success on port %s\n", port_name);
    print_bar();

    return 0;

HELL:
 
    print_bar();
    return -1;
}

I am working with an adapter using an ASIX chip set and one using an FTDI chip set, the FTDI based device has no problems but the other simply returns -1 from ioctl when i try to set it up with the first TIOCSSERIAL command. So is there a way to detect if the TIOCSSERIAL command not is supported by the used driver?

PS! I am using the label HELL: as a common return point on error of my test program ;-)

Answer 1

USB-to-serial adapters do not support nor need those setserial ioctls.

If you want to set a custom speed on a USB-to-serial adapter, you should use the new TCSETS2, TCSETSW2 and TCSETSF2 ioctls, which take a struct termios2 where you should set the BOTHER flag in .c_cflag and use the .c_ispeed and .c_ospeed fields directly. Look at /usr/include/asm-generic/termbits.h.

No divisor setting or other such thing is necessary.

The last time I did that I had to use some #define kludges in order to be able to include those headers (they conflict with the termios headers from glibc).

Example wrappers (from some old sources, UNTESTED):

#include <sys/ioctl.h>
#define termios termios_HIDE
#define termio termio_HIDE
#define winsize winsize_HIDE
#include <asm/termios.h>
#undef termios
#undef termio
#undef winsize
#define termios termios2
#define tcsetattr tcsetattr2
#define tcgetattr tcgetattr2

int tcsetattr2(int fd, int act, struct termios *ts){
        return act == TCSANOW ? ioctl(fd, TCSETS2, ts) :
                act == TCSADRAIN ? ioctl(fd, TCSETSW2, ts) :
                ioctl(fd, TCSETSF2, ts);
}
int tcgetattr2(int fd, struct termios *ts){
        return ioctl(fd, TCGETS2, ts);
}
int cfsetspeed(struct termios *ts, speed_t s){
        ts->c_cflag |= BOTHER;
        ts->c_ispeed = ts->c_ospeed = s;
        return 0;
}