termios: document callback more clearly

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/tty.txt b/Documentation/tty.txt
index 048a8762cfb5..8e65c4498c52 100644
--- a/Documentation/tty.txt
+++ b/Documentation/tty.txt
@@ -132,6 +132,14 @@ set_termios()		Notify the tty driver that the device's termios
                         tty->termios. Previous settings should be passed in
                         the "old" argument.
 
+                        The API is defined such that the driver should return
+                        the actual modes selected. This means that the
+                        driver function is responsible for modifying any
+                        bits in the request it cannot fulfill to indicate
+                        the actual modes being used. A device with no
+                        hardware capability for change (eg a USB dongle or
+                        virtual port) can provide NULL for this method.
+
 throttle()              Notify the tty driver that input buffers for the
                         line discipline are close to full, and it should
                         somehow signal that no more characters should be