null TTY error

Hi all,

Porting the kernel 2.4.1 to my MIPS embedded board,
i encountered a TTY problem.
When the printf is putted in a large loop, such as a 100 times loop, 
the kernel error message "Warning: null TTY for (ff:ff) in tty_write" appears.
However, if just one or two printf are used, everything is fine.
Does this problem concern with my serial driver?
I wrote my serial driver based on driver/char/serial.c.
Can someone give me a hint that where should be checked over?

Thanks.