Re: [Xenomai] xddp port questions

[Philippe Gerum <rpm@xenomai.org>]

On 01/20/2014 05:18 PM, Leopold Palomo-Avellaneda wrote:
> A Dimarts, 14 de gener de 2014, Philippe Gerum va escriure:
>> 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.
>
> Ok,
>
> in the example the same thread open a device for write and read from the same
> device. From your answer about the endpoints, I understand that when the NRT
> open for read, read from the RT world, and when the NRT writes to the RT
> world.
>
> So, could I have a NRT function (thread) that open the device to just read
> from the RT world and _another_ that send to the RT world?
>
> Because I have to one the same device, and I don't know that I could do it at
> the same time.
>

You can both send-receive data to/from the same socket descriptor 
RT-wise, and read-write from/to the same /dev/rtp* descriptor NRT-wise. 
Message pipes are bi-directional by design, on both ends. In the XDDP 
case, traffic is routed as follows:

- RT-writes -> NRT-reads
- RT-reads <- NRT-writes

So, fd = open("/dev/rtpX", ...) => read(fd, and write(fd, are allowed
     s = socket(AF_RTIPC, ...) => sendmsg(s,  and recv(s, are allowed.

-- 
Philippe.