Re: [Xenomai-help] [RT_PIPE] - User space read() not waking up

[Philippe Gerum <rpm@xenomai.org>]

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.

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
}
-- 
Philippe.