Question

I have a USB device I’m trying to communicate with over a virtual serial port provided by the ftdi_sio kernel module. However, I’m having some trouble setting the baud rate of the port to 14400:

• termios.h doesn’t specify a constant for 14400, so I can’t use cfsetispeed and cfsetospeed.
• In the source for the ftdi_sio kernel module, baud base is set to 24000000 and there doesn’t seem to be a way to change it. This means I can’t use a custom divisor with the TIOCSSERIAL ioctl and get a 14400 baud rate that way.
• The module source has a comment making it sound like setting the alt_speed member of the tty_struct structure for the port to 14400 would do what I want, but there doesn’t seem to be any way to set it to 14400 given the existing interfaces.

Does anyone have any ideas about this? It’d be pretty easy to fix this by hacking up the kernel module, but I’m really looking for a solution that doesn’t require kernel changes.

Answer 1

You can’t change baud base, I suppose it is hardware related. So messing with the module won’t do you any good. In your third point you only talk about the first method proposed for setting a custom baudrate, where you need to access the tty->alt_speed. It seems there is no interface to directly set tty struct from userspace, at least not with the ftdi_sio driver.
However, there is another method explained in the comments :

     * 3. You can also set baud rate by setting custom divisor as follows
     *    - set tty->termios->c_cflag speed to B38400
     *    - call TIOCSSERIAL ioctl with (struct serial_struct) set as
     *      follows:
     *      o flags & ASYNC_SPD_MASK == ASYNC_SPD_CUST
     *      o custom_divisor set to baud_base / your_new_baudrate

Did you try it ?