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

[Kashyap Chamarthy <kchamart@redhat.com>]

On Tue, Feb 16, 2016 at 04:03:01PM +0000, Peter Maydell wrote:
> 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...
> ---

Thanks for following up with the clarifying write-up.

>  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.

Just noting for convenience here, looking at an old commit[1 that
notes the syntax for multiplexing 'stdio', which you mention above in
prose:

    [...]
    Muxing can be done this way:
    
        -chardev stdio,id=mux,mux=on
        -serial chardev:mux
        -monitor chardev:mux"
    [...]

[1] http://git.qemu.org/?p=qemu.git;a=commitdiff;h=c845f40

> +
>  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}

Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>

-- 
/kashyap