int copy_from_read_buf (struct tty_struct * tty, unsigned char __user ** b, size_t * nr);
Helper function to speed up read_chan. It is only called when ICANON is off; it copies characters straight from the tty queue to user space directly. It can be profitably called twice; once to drain the space from the tail pointer to the (physical) end of the buffer, and once to drain the space from the (physical) beginning of the buffer to head pointer.
Called under the tty->atomic_read sem and with TTY_DONT_FLIP set