+/**
+ * process_echoes - write pending echo characters
+ * @tty: terminal device
+ *
+ * Write previously buffered echo (and other ldisc-generated)
+ * characters to the tty.
+ *
+ * Characters generated by the ldisc (including echoes) need to
+ * be buffered because the driver's write buffer can fill during
+ * heavy program output. Echoing straight to the driver will
+ * often fail under these conditions, causing lost characters and
+ * resulting mismatches of ldisc state information.
+ *
+ * Since the ldisc state must represent the characters actually sent
+ * to the driver at the time of the write, operations like certain
+ * changes in column state are also saved in the buffer and executed
+ * here.
+ *
+ * A circular fifo buffer is used so that the most recent characters
+ * are prioritized. Also, when control characters are echoed with a
+ * prefixed "^", the pair is treated atomically and thus not separated.
+ *
+ * Locking: output_lock to protect column state and space left,
+ * echo_lock to protect the echo buffer
+ */
+
+static void process_echoes(struct tty_struct *tty)
+{
+ int space, nr;
+ unsigned char c;
+ unsigned char *cp, *buf_end;
+
+ if (!tty->echo_cnt)
+ return;
+
+ mutex_lock(&tty->output_lock);
+ mutex_lock(&tty->echo_lock);
+
+ space = tty_write_room(tty);
+
+ buf_end = tty->echo_buf + N_TTY_BUF_SIZE;
+ cp = tty->echo_buf + tty->echo_pos;
+ nr = tty->echo_cnt;
+ while (nr > 0) {
+ c = *cp;
+ if (c == ECHO_OP_START) {
+ unsigned char op;
+ unsigned char *opp;
+ int no_space_left = 0;
+
+ /*
+ * If the buffer byte is the start of a multi-byte
+ * operation, get the next byte, which is either the
+ * op code or a control character value.
+ */
+ opp = cp + 1;
+ if (opp == buf_end)
+ opp -= N_TTY_BUF_SIZE;
+ op = *opp;
+
+ switch (op) {
+ unsigned int num_chars, num_bs;
+
+ case ECHO_OP_ERASE_TAB:
+ if (++opp == buf_end)
+ opp -= N_TTY_BUF_SIZE;
+ num_chars = *opp;
+
+ /*
+ * Determine how many columns to go back
+ * in order to erase the tab.
+ * This depends on the number of columns
+ * used by other characters within the tab
+ * area. If this (modulo 8) count is from
+ * the start of input rather than from a
+ * previous tab, we offset by canon column.
+ * Otherwise, tab spacing is normal.
+ */
+ if (!(num_chars & 0x80))
+ num_chars += tty->canon_column;
+ num_bs = 8 - (num_chars & 7);
+
+ if (num_bs > space) {
+ no_space_left = 1;
+ break;
+ }
+ space -= num_bs;
+ while (num_bs--) {
+ tty_put_char(tty, '\b');
+ if (tty->column > 0)
+ tty->column--;
+ }
+ cp += 3;
+ nr -= 3;
+ break;
+
+ case ECHO_OP_SET_CANON_COL:
+ tty->canon_column = tty->column;
+ cp += 2;
+ nr -= 2;
+ break;
+
+ case ECHO_OP_MOVE_BACK_COL:
+ if (tty->column > 0)
+ tty->column--;
+ cp += 2;
+ nr -= 2;
+ break;
+
+ case ECHO_OP_START:
+ /* This is an escaped echo op start code */
+ if (!space) {
+ no_space_left = 1;
+ break;
+ }
+ tty_put_char(tty, ECHO_OP_START);
+ tty->column++;
+ space--;
+ cp += 2;
+ nr -= 2;
+ break;
+
+ default:
+ /*
+ * If the op is not a special byte code,
+ * it is a ctrl char tagged to be echoed
+ * as "^X" (where X is the letter
+ * representing the control char).
+ * Note that we must ensure there is
+ * enough space for the whole ctrl pair.
+ *
+ */
+ if (space < 2) {
+ no_space_left = 1;
+ break;
+ }
+ tty_put_char(tty, '^');
+ tty_put_char(tty, op ^ 0100);
+ tty->column += 2;
+ space -= 2;
+ cp += 2;
+ nr -= 2;
+ }
+
+ if (no_space_left)
+ break;
+ } else {
+ if (O_OPOST(tty) &&
+ !(test_bit(TTY_HW_COOK_OUT, &tty->flags))) {
+ int retval = do_output_char(c, tty, space);
+ if (retval < 0)
+ break;
+ space -= retval;
+ } else {
+ if (!space)
+ break;
+ tty_put_char(tty, c);
+ space -= 1;
+ }
+ cp += 1;
+ nr -= 1;
+ }
+
+ /* When end of circular buffer reached, wrap around */
+ if (cp >= buf_end)
+ cp -= N_TTY_BUF_SIZE;
+ }
+
+ if (nr == 0) {
+ tty->echo_pos = 0;
+ tty->echo_cnt = 0;
+ tty->echo_overrun = 0;
+ } else {
+ int num_processed = tty->echo_cnt - nr;
+ tty->echo_pos += num_processed;
+ tty->echo_pos &= N_TTY_BUF_SIZE - 1;
+ tty->echo_cnt = nr;
+ if (num_processed > 0)
+ tty->echo_overrun = 0;
+ }
+
+ mutex_unlock(&tty->echo_lock);
+ mutex_unlock(&tty->output_lock);
+
+ if (tty->ops->flush_chars)
+ tty->ops->flush_chars(tty);
+}