qemu-options.hx: Improve documentation of chardev multiplexing mode

The current documentation of chardev mux=on is rather brief and opaque;
expand it to hopefully be a bit more helpful.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-Id: <1455643738-6068-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qemu-doc.texi

@@ -158,7 +158,8 @@ TODO (no longer available)
 * pcsys_introduction:: Introduction
 * pcsys_quickstart::   Quick Start
 * sec_invocation::     Invocation
-* pcsys_keys::         Keys
+* pcsys_keys::         Keys in the graphical frontends
+* mux_keys::           Keys in the character backend multiplexer
 * pcsys_monitor::      QEMU Monitor
 * disk_images::        Disk Images
 * pcsys_network::      Network emulation
@@ -272,7 +273,7 @@ targets do not need a disk image.
 @c man end
 
 @node pcsys_keys
-@section Keys
+@section Keys in the graphical frontends
 
 @c man begin OPTIONS
 
@@ -322,15 +323,23 @@ Toggle mouse and keyboard grab.
 In the virtual consoles, you can use @key{Ctrl-Up}, @key{Ctrl-Down},
 @key{Ctrl-PageUp} and @key{Ctrl-PageDown} to move in the back log.
 
-@kindex Ctrl-a h
-During emulation, if you are using the @option{-nographic} option, use
-@key{Ctrl-a h} to get terminal commands:
+@c man end
+
+@node mux_keys
+@section Keys in the character backend multiplexer
+
+@c man begin OPTIONS
+
+During emulation, if you are using a character backend multiplexer
+(which is the default if you are using @option{-nographic}) then
+several commands are available via an escape sequence. These
+key sequences all start with an escape character, which is @key{Ctrl-a}
+by default, but can be changed with @option{-echr}. The list below assumes
+you're using the default.
 
 @table @key
 @item Ctrl-a h
 @kindex Ctrl-a h
-@item Ctrl-a ?
-@kindex Ctrl-a ?
 Print this help
 @item Ctrl-a x
 @kindex Ctrl-a x
@@ -346,10 +355,11 @@ Toggle console timestamps
 Send break (magic sysrq in Linux)
 @item Ctrl-a c
 @kindex Ctrl-a c
-Switch between console and monitor
+Rotate between the frontends connected to the multiplexer (usually
+this switches between the monitor and the console)
 @item Ctrl-a Ctrl-a
-@kindex Ctrl-a a
-Send Ctrl-a
+@kindex Ctrl-a Ctrl-a
+Send the escape character to the frontend
 @end table
 @c man end
 

qemu-options.hx

@@ -2162,8 +2162,49 @@ 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.
-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.
+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:
+
+@example
+-chardev stdio,mux=on,id=char0 \
+-mon chardev=char0,mode=readline,default \
+-serial chardev:char0 \
+-serial chardev:char0
+@end example
+
+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:
+
+@example
+-chardev stdio,mux=on,id=char0 \
+-mon chardev=char0,mode=readline,default \
+-parallel chardev:char0 \
+-chardev tcp,...,mux=on,id=char1 \
+-serial chardev:char1 \
+-serial chardev:char1
+@end example
+
+When you're using a multiplexed character device, some escape sequences are
+interpreted in the input. @xref{mux_keys, Keys in the character backend
+multiplexer}.
+
+Note that some other command line options may implicitly create multiplexed
+character backends; for instance @option{-serial mon:stdio} creates a
+multiplexed stdio backend connected to the serial port and the QEMU monitor,
+and @option{-nographic} also multiplexes the console and the monitor to
+stdio.
+
+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}