Ionotify on a serial por

I’d like to be notified when a specified number of bytes is available for
reading (a packet arrives) on a serial port managed by devc-ser8250. I can’t
use readcond because it’s timeout is too large. So I tried to use ionotify,
but can’t find a way to specify a number of bytes. I supposed that the
number corresponds to MIN in termio structure, but found strange results:
the value specified and succesfully tcsetattr’d doesn’t shows zero on the
next call to tcgetattr or stty from shell. Switching to a blocked mode
doesn’t help. I’m really puzzled.