- struct tty_struct *to = tty->link;
- struct tty_ldisc *ld;
- int count = 0;
-
- /* We should get the line discipline lock for "tty->link" */
- if (!to)
- return 0;
- /* We cannot take a sleeping reference here without deadlocking with
- an ldisc change - but it doesn't really matter */
- ld = tty_ldisc_ref(to);
- if (ld == NULL)
- return 0;
-
- /* The ldisc must report 0 if no characters available to be read */
- if (ld->ops->chars_in_buffer)
- count = ld->ops->chars_in_buffer(to);
-
- tty_ldisc_deref(ld);
-
- if (tty->driver->subtype == PTY_TYPE_SLAVE)
- return count;
-
- /* Master side driver ... if the other side's read buffer is less than
- * half full, return 0 to allow writers to proceed; otherwise return
- * the count. This leaves a comfortable margin to avoid overflow,
- * and still allows half a buffer's worth of typed-ahead commands.
- */
- return (count < N_TTY_BUF_SIZE/2) ? 0 : count;