Hey, thanks for the response!
So, I'm doing something similar - I have an ARM communicating with the MIPS processor over /dev/ttyS1. I did some more testing, and I found that the MIPS seems to be running about 10% slower than specified baud rate when set to 921600. E.g. when set to 921600 on the MIPS (using stty) I have to set the baud rate to 829440 on the ARM processor for reliable data transmission.
Your links are helpful, thanks! I'm glad at least the ARM has a very flexible baud rate generator.
Is 1000000 baud still the max you were able to set? It's not clear to me what valid baud rates are allowed on the MIPS, but I could find 921600 (829440) acceptable.