tty: Simplify tty buffer/ldisc interface with helper function

Ldisc interface functions must be called with interrupts enabled.
Separating the ldisc calls into a helper function simplies the
eventual removal of the spinlock.

Note that access to the buf->head ptr outside the spinlock is
safe here because;
* __tty_buffer_flush() is prevented from running while buffer work
  performs i/o,
* tty_buffer_find() only assigns buf->head if the flip buffer list
  is empty (which is never the case in flush_to_ldisc() since at
  least one buffer is always left in the list after use)
Access to the read index outside the spinlock is safe here for the
same reasons.

Update the buffer's read index _after_ the data has been received
by the ldisc.
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_buffer.c

@@ -403,6 +403,18 @@ int tty_prepare_flip_string_flags(struct tty_port *port,
 EXPORT_SYMBOL_GPL(tty_prepare_flip_string_flags);
 
 
+static int
+receive_buf(struct tty_struct *tty, struct tty_buffer *head, int count)
+{
+	struct tty_ldisc *disc = tty->ldisc;
+
+	count = min_t(int, count, tty->receive_room);
+	if (count)
+		disc->ops->receive_buf(tty, head->char_buf_ptr + head->read,
+				       head->flag_buf_ptr + head->read, count);
+	head->read += count;
+	return count;
+}
 
 /**
  *	flush_to_ldisc
@@ -438,8 +450,6 @@ static void flush_to_ldisc(struct work_struct *work)
 		struct tty_buffer *head;
 		while ((head = buf->head) != NULL) {
 			int count;
-			char *char_buf;
-			unsigned char *flag_buf;
 
 			count = head->commit - head->read;
 			if (!count) {
@@ -449,16 +459,10 @@ static void flush_to_ldisc(struct work_struct *work)
 				tty_buffer_free(port, head);
 				continue;
 			}
-			if (!tty->receive_room)
-				break;
-			if (count > tty->receive_room)
-				count = tty->receive_room;
-			char_buf = head->char_buf_ptr + head->read;
-			flag_buf = head->flag_buf_ptr + head->read;
-			head->read += count;
 			spin_unlock_irqrestore(&buf->lock, flags);
-			disc->ops->receive_buf(tty, char_buf,
-							flag_buf, count);
+
+			count = receive_buf(tty, head, count);
+
 			spin_lock_irqsave(&buf->lock, flags);
 			/* Ldisc or user is trying to flush the buffers.
 			   We may have a deferred request to flush the
@@ -469,7 +473,8 @@ static void flush_to_ldisc(struct work_struct *work)
 				clear_bit(TTYP_FLUSHPENDING, &port->iflags);
 				wake_up(&tty->read_wait);
 				break;
-			}
+			} else if (!count)
+				break;
 		}
 		clear_bit(TTYP_FLUSHING, &port->iflags);
 	}