[PATCH] n_gsm/documentation: Add some words about DCD signal

[Lars Poeschel]

From: Lars Poeschel <poeschel@xxxxxxxxxxx>

Add some words about modem control lines, DCD signal and that
an open may block to the n_gsm documentation.

Signed-off-by: Lars Poeschel <poeschel@xxxxxxxxxxx>
---
 Documentation/serial/n_gsm.txt | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/serial/n_gsm.txt b/Documentation/serial/n_gsm.txt
index a5d9112..089b934 100644
--- a/Documentation/serial/n_gsm.txt
+++ b/Documentation/serial/n_gsm.txt
@@ -74,6 +74,12 @@ done
 for example, it's possible :
 - and to use gnokii to send / receive SMS on ttygsm1
 - to use ppp to establish a datalink on ttygsm2
+- If you want to use a virtual muxed channel in your own programs, be aware
+  that it respects the modem control lines. If you open the virtual port it
+  will block, if it is not connected to a carrier (DCD signal missing). You
+  could open the virtual port with O_NDELAY flag and do normal AT commands
+  to the modem without a carrier. You can change to blocking mode later if
+  you need this with fcntl F_SETFL/F_GETFL.
 
 6- first close all virtual ports before closing the physical port.
 
-- 
1.8.5.2

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