[etherlab-users] Usage of EtherCAT-Master Userspace-library breaks hard realtime execution under RTAI/LXRT

Florian Pose fp at igh-essen.com
Wed Apr 14 15:13:21 CEST 2010


On Tue, Apr 06, 2010 at 05:50:48PM +0200, J├╝rgen Kunz wrote:
> There are two points which are breaking realtime
> execution under rtai/lxrt: 1. using ioctl (see
> http://www.rtai.dk/cgi-bin/gratiswiki.pl?LXRT_FAQs) and 2. using kernel
> semaphores to lock the above mentioned kernel function calls, which are
> not locked if they are used in kernel space directly. I commented out
> the usage of the kernel semaphores in the relating ioctl-functions in
> kernel space, but that makes no difference that the hard-realtime-thread
> is pushed back to soft realtime execution.
> So, why are kernel semaphores being used here? It is mentioned in the
> documentation that usage of standard kernel semaphores are not
> sufficient for providing a correct locking mechanism if realtime
> extensions like rtai and xenomai are used and so the application which
> has requested the master has to provide the appropriate locking
> mechanism. Does an implementation of the kernel calls exist which can
> guarantee hard realtime execution in user space? Or do I have to
> implement the corresponding LXRT-calls by myself?

the user-space library is not designed for the use with LXRT etc.. The
kernel itself has to be realtime-capable (for example with RT-PREEMPT).
I heard of people who wrote a wrapper layer for ethercat API functions,
but for an older interface version.

If you really want tu use the userspace API via LXRT, you have to
implement this kind of wrapper class to avoid the ioctl() calls.

Best regards,
Florian Pose


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