Frank van der Linden <fvdl@wasabisystems.com>  wrote:
 > You can only really use a speed > 115200 if the serial chip is clocked
 > with a frequency high enough to deal with it. If you have a 16x50 with
 > a higher clock, you can change COM_FREQ in sys/dev/ic/comreg.h to
 > match it. There's currently no way to change that at runtime.
 > 
 > The hardware must actually allow it though. You can't make a 16550
 > with a crystal at 1.8432 Mhz do 230400.

Ok. I've gotten to the bottom of it.  They're using a 7.3728Mhz oscillator
on the card so I'll have to change COM_FREQ.  

However, that means I'll either have to boot a custom kernel every time
I want to use this card on my Jornada _or_ divide my baud rates by 4
whenever I want to use some other serial device.

What's the right way to solve this?