Re: [Xenomai] xddp port questions

[Philippe Gerum <rpm@xenomai.org>]

On 01/14/2014 01:15 PM, Leopold Palomo-Avellaneda wrote:
> Hi,
>
> in the example xddp-label a two realtime task are connected with a non-
> realtime time. They share the same port where one rt task receive from the
> non-rt task and the other send to rt-task.
>
> The regular thread read and write in the same devname
> (/proc/xenomai/registry/rtipc/xddp/XDDP_PORT_LABEL). One rt thread listen from
> that port.
>
> I don't understand how is possible that both (non-rt and rt threads) listen
> from the same port and could not have collisions  if two threads (non-rt and
> rt) try to write in the same port.
>
> Could not be better to separate it?
>

No, that would defeat the purpose of the illustration.

XDDP is a wrapper over Xenomai's message pipe support, offering a 
socket-based interface to applications. Each XDDP port is mapped to a 
given /dev/rtp device minor, but the communication endpoints between RT 
and NRT are different internally.

[XDDP-port] <---> xnpipe #<port>
                      ^
                      |
                      |  * input queue:  /dev/rtp -> xnpipe
                      |  * output queue: xnpipe -> /dev/rtp
                      |
                      v
                /dev/rtp<port>

So, when NRT reads from /dev/rtp<port>, it does not actually listen to 
the same endpoint/queue than RT, because message pipes are 
bi-directional. Likewise, NRT and RT never write to the same queue, 
since the purpose of message pipes is to cross the RT/NRT domain boundary.

-- 
Philippe.