Philippe Gerum wrote:
On Sat, 2007-03-24 at 11:20 +0100, Dmitry Adamushko wrote:
3) Pipes seemed to be a good way to transfer data from kernel
space.
What other (maybe more deterministic) way would you suggest?
I'm not
asking for a complete program architecture here just a half
dozen words
describing how you would have done it.
rt_queue_*
I would add that if you really want to keep the pipe semantics for
exchanging data without crossing domains between real-time tasks, then
you may access the real-time API of message pipes from user-space too.
i.e.
rt_pipe_create/bind/delete/read/write/stream() are all available from a
user-space application, in which case data exchange will happen in
primary mode.
Now, that is interesting. I was left under the impression that in user
space, the only option to access pipes was using regular
open()/read()/write().
I guess I overlooked something in the documentation. Thank you for
pointing this out.
To sum up, given that you want to create a pipe in kernel space, to
exchange data with a user-space program, the options with Xenomai's
message pipes are:
1) Implementing a real-time communication path between two Xenomai
tasks, crossing the kernel/user-space boundary without leaving the
deterministic primary domain: you have to create the kernel-side
endpoint in your module, then bind the user-space endpoint to the latter
using the proper interface, as shown below in the REALTIME_TO_REALTIME
illustration.
2) Implementing a communication path between a real-time Xenomai task in
kernel space, and a regular user-space program: you still have to create
the kernel-side endpoint in your module, then use the standard
open/read/write routines to access to the pipe channel. This is what you
are doing right now.
kernel-side:
void foo_task(void)
{
for (;;) {
rt_pipe_read/write(&pipe_desc, ...);
}
}
int init_module(void)
{
rt_pipe_create(&pipe_desc, "channel", ...);
}
user-space side:
int main(int argc, char *argv[])
{
#ifdef REALTIME_TO_REALTIME
/* promote main() to the Xenomai real-time space */
rt_task_shadow(&task_desc, __FUNCTION__, ...);
/* bind to the pipe created in kernel space */
rt_pipe_bind(&pipe_desc, "channel", TM_INFINITE);
for (;;) {
rt_pipe_read/write(&pipe_desc, "channel", ...);
}
#else /* REALTIME_TO_REGULAR */
fd = open("/proc/xenomai/registry/native/pipes/channel",O_RDWR);
for (;;) {
read/write(fd, ...);
}
#endif
}
That looks like a pretty good solution. I'll be able to test this out on
Monday. I'll keep the list posted about my results.
Thanks a lot for your enlightening comments / example. It's very much
appreciated.
Viktor STARK
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