Dear hackers, I've been passing last month seeking for the best way to implement Powerlink with Xenomai. I still didn't find the absolute truth, and would like to confront my point of view.
In short, Powerlink is a kind of CANopen over polled ethernet. Powerlink re-use a subset of CANopen concepts for high level communications, whereas on the low level, a single master cyclically poll each node for data. As you can imagine, communication cycle is seriously influenced by each controlled node latency answering poll requests from master. My intention is to write an efficient Powerlink implementation for Xenomai, but suitable for later reuse in future Real Time Linux implementation such as RT-PREEMPT. Discussing with RT-PREEMT gurus, I understood that re-using native Linux queues could never be a solution, as those queues induce way too long and unpredictable latencies for such real-time ethernet (those latencies also apply for SocketCan). Some software hook at ethernet driver level will be necessary to reroute RT-related packets directly to the concerned RT stack. With this approach, the whole stack would have to stay kernel side to reduce latency in answering poll requests. Implementing the whole CANopen stack kernel side is generally a bad idea, as it complicate access to object dictionary from application, usually in user space. RTnet proves that Real-Time ethernet can be implemented through the socket paradigm. I believe that Time Division Medium Access (TDMA) of RTnet could be interchanged with polled ethernet medium access discipline from Powerlink. Thus, CANopen implementation part could stay user side, and could pre-fill answers to poll requests, avoiding long latencies. But what is the future of RTnet ? Will it be ported to RT-PREEMPT or will it enter official Xenomai tree ? Will available RTnet's drivers set continue to grow ? Would it be theoretically possible for RTnet to support Linux ethernet drivers through a hook mechanism as mentioned in previous paragraph ? My feeling is that, yes, we should try to fit implementation of Powerlink data link layer inside RTnet as another MAC discipline, and use a modified version of CanFestival for CANopen aspects, keeping it a user space library. Please comment. Best regards, Edouard -- LOLITech 24, rue Pierre Evrat 88100 Saint-Dié des Vosges tel: +33 3 29 57 60 42 mob: +33 6 10 49 76 41 [email protected] http://www.lolitech.fr _______________________________________________ Xenomai-help mailing list [email protected] https://mail.gna.org/listinfo/xenomai-help
