Serial driver ignores custom baud rates.

[rfmerrill]

Linux has an (admittedly deprecated) interface from setting custom baud rates, but it appears that some drivers, including the raspberry pi's, silently ignore it.

This is a bit of a shame because the chip is actually quite flexible with baud rates.

I came upon this issue while trying to get the Pi to receive DMX. DMX is a mostly-standard serial interface that uses the unfortunate baud rate of 250000 bps.

I set the UART clock to 4 megahertz, which would make 250000 an easy division (it's just /16), but the documented method for setting custom baud rates just did nothing. What's interesting is that the amba-pl010 driver appears to support it but the amba-pl011 driver does not.

In a last-ditch attempt to get it working, I set the serial port up for 230400, which worked fine, and then I manually poked the right divisor into the UART's register and the driver continued on happily.

If you guys don't want to modify the driver, is there a way I can completely disable it without recompiling the kernel? That way I could maybe just bang on it the ugly way or insert another driver that does the same thing.

[popcornmix]

What is the deprecated interface you are referring to?

I do have a patch that enables arbitrary baud rates. It seems to enabled CBAUDEX:
tios->c_cflag |= CBAUDEX;
and then write the baud rate to:
/proc/uart_cur_freq
which doesn't seem very standard. What's your scheme like?

[rfmerrill]

setting custom_divisor with the TIOCSSERIAL ioctl

[popcornmix]

Can you point out where amba-pl010.c is handling this, and amba-pl011.c is not?

You can vary the clock given to the uart with config.txt parameter init_uart_clock. See:
http://www.siliconstuff.com/2012/08/serial-port-midi-on-raspberry-pi.html

[rfmerrill]

amba-pl010.c calls uart_get_divisor to get the divisor, while -pl011 calculates it by hand. uart_get_divisor will return the custom divisor if it's set.

[popcornmix]

Have you tried patching amba-pl011.c?

[rfmerrill]

Sorry, no I have not. Been a while since I recompiled a kernel, haven't had the time to attempt it.

[RishiGupta12]

I was able to set 25000 custom baudrate on FT232R chip http://www.embeddedunveiled.com/set-custom-baud-rate. I am now going to try it on raspberry pi as a part of porting this library to raspberry pi.