serial: doc: .(un)throttle() depends on hardware assisted flow control

Document that .throttle() and .unthrottle() are relevant only if
hardware assisted flow control is enabled.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/Documentation/serial/driver b/Documentation/serial/driver
index 03369d076ca2..03b703cf9318 100644
--- a/Documentation/serial/driver
+++ b/Documentation/serial/driver
@@ -135,6 +135,7 @@ hardware.
         Notify the serial driver that input buffers for the line discipline are
         close to full, and it should somehow signal that no more characters
         should be sent to the serial port.
+        This will be called only if hardware assisted flow control is enabled.
 
         Locking: none.
 
@@ -142,6 +143,7 @@ hardware.
         Notify the serial driver that characters can now be sent to the serial
         port without fear of overrunning the input buffers of the line
         disciplines.
+        This will be called only if hardware assisted flow control is enabled.
 
         Locking: none.