Re: [Xenomai] xddp port questions

["Leopold Palomo-Avellaneda" <leo@alaxarxa.net>]

A Dilluns, 20 de gener de 2014, Philippe Gerum va escriure:
> 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.
> 
Sorry if I have not been clear or I don't understand you. For design I would 
like to make a thread that:

fd = open("/dev/rtpX", ...) => read(fd, ...

to read

and another thread to

fd = open("/dev/rtpX", ...) => write(fd, ...

so I open TWO times the /dev/rtpX, one for read and another for write, because 
I would like to use it in two different parts, and I don't know if it's 
allowed.

I understand that the RT part and the NRT part open the same /dev/rtp*, but I 
don't know if several NRT (or RT) could open the same open descriptor at the 
same time.

Regards,

Leopold



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Linux User 152692
Catalonia
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