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>

diff --git a/Documentation/tty.txt b/Documentation/tty.txt
index 048a8762cfb548fbe2b2078ea8527a7a9c50bc01..8e65c4498c521259d93061e0a94a2d3652eaef98 100644 (file)
--- 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