[Qemu-devel] [PATCH] qemu-options.hx: Improve documentation of chardev multiplexing mode

[Peter Maydell <peter.maydell@linaro.org>]

The current documentation of chardev mux=on is rather brief and opaque;
expand it to hopefully be a bit more helpful.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
There was some discussion on #qemu yesterday evening about multiplexing,
and "make the docs a bit less confusing" was one suggestion...
---
 qemu-options.hx | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/qemu-options.hx b/qemu-options.hx
index 2f0465e..6b46dd4 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -2162,8 +2162,26 @@ All devices must have an id, which can be any string up to 127 characters long.
 It is used to uniquely identify this device in other command line directives.
 
 A character device may be used in multiplexing mode by multiple front-ends.
+Specify @option{mux=on} to enable this mode.
+A multiplexer is a "1:N" device, and here the "1" end is your specified chardev
+backend, and the "N" end is the various parts of QEMU that can talk to a chardev.
+If you create a chardev with @option{id=myid} and @option{mux=on}, QEMU will
+create a multiplexer with your specified ID, and you can then configure multiple
+front ends to use that chardev ID for their input/output. Up to four different
+front ends can be connected to a single multiplexed chardev. (Without
+multiplexing enabled, a chardev can only be used by a single front end.)
+For instance you could use this to allow a single stdio chardev to be used by
+two serial ports and the QEMU monitor.
+
 The key sequence of @key{Control-a} and @key{c} will rotate the input focus
-between attached front-ends. Specify @option{mux=on} to enable this mode.
+of a multiplexed character device between attached front-ends.
+
+You can have more than one multiplexer in a system configuration; for instance
+you could have a TCP port multiplexed between UART 0 and UART 1, and stdio
+multiplexed between the QEMU monitor and a parallel port.
+
+There is currently no support for multiplexing in the other direction
+(where a single QEMU front end takes input and output from multiple chardevs).
 
 Every backend supports the @option{logfile} option, which supplies the path
 to a file to record all data transmitted via the backend. The @option{logappend}
-- 
1.9.1