Cannot change baud rate on usb serial device

I think I found the solution to this, in case anyone googles it in the future. My 10.04 did not load usbserial by default, so I had to do it with modprobe usbserial vendor=0x0403 product=0x6001, which I added to /etc/modules. After I upgraded, my 12.04 loads the ftdi_sio driver by default, which is for the embedded device I am using. I didn't change my /etc/modules back, so usbserial was being loaded too, and probably causing some sort of conflict. I removed usbserial from /etc/modules and rebooted, and it worked.

If I were really confident in this solution I would confirm it by rebooting my machine again and seeing if the serial port came up properly - but I would feel pretty silly if I rebooted and lost the connection, since I do need to get work done while I have it.

USB to serial devices like the Prolific or FTDI series don't follow any official specification. Instead, they use two bulk endpoints (one for output, one for input) with specific vendor-specific control messages used to configure things like baudrate. Thus, it's not too much of a surprise if something like the generic usbserial driver has trouble setting baudrates or doing other configuration tasks on a FTDI chip. As you noticed, using ftdi_sio will work much better.

Even 10.04 should be loading ftdi_sio by default, as ftdi_sio has been hardcoded to handle 4003:6001 (FT232) devices for nearly as long as Ubuntu has existed. My guess is that you were loading two drivers that were both registered to handle 4003:6001 -- one by design, one due to your module parameters. Changing your kernel or even the phase of the moon could mean that one happens to get loaded before the other -- and if usbserial gets loaded first, you will not be able to set baudrate. You should remove your manual usbserial configuration, and let ftdi_sio handle the device. It will work on both 10.04 and 12.04.