Re: [Xenomai] POSIX application running under xenomai -- what do wrapped functions do?

[Philippe Gerum <rpm@xenomai.org>]

On 07/25/2014 04:54 PM, Steve M. Robbins wrote:
> On July 23, 2014 10:37:35 PM Philippe Gerum wrote:
> 
>> RTIPC/xddp and the Message pipe services are based on the very same code
>> in the Xenomai core, it's only a matter of differing interfaces. Over
>> 2.5.6, I would go for the pipes, since the xddp driver in that release
>> might lack a couple of fixes which were merged into 2.6.
> 
> OK, we are now looking a bit more closely at the 2.5 Message Pipe docs and my colleague noticed a small quirk.  
> 
> For rt_pipe_read(), the doc [1] says:
> 
> 		Environments:
> 
> 		This service can be called from:
> 
> 		 *    Kernel module initialization/cleanup code

not related, but the TM_NONBLOCK clause applies above as well.

> 		 *    Interrupt service routine only if timeout is equal to TM_NONBLOCK.
> 
> 		 *    Kernel-based task
> 		 *    User-space task (switches to primary mode)
> 
> So a literal reading of the last two bullet points suggest that a call from kernel-based task never switches to primary mode while a call from the user-space task always does.  Naively that seems surprising.  Is it accurate?  
> 

Because a kernel-based Xenomai thread can only run in primary mode over
the Xenomai 2.x architecture, so the mode switch does not apply in essence.

> Secondly, for a user-space task: will it mode switch if there is data available?  If we use TM_NONBLOCK in a user space task, can we avoid the mode switch?
> 

It will mode switch regardless of whether data is available, the
transition happens in the early syscall path before we know about the
pipe status. TM_NONBLOCK won't help here.

> Finally, the function rt_pipe_receive() is pretty similar to rt_pipe_read() so much so that I would guess one is implemented using the other.  Thus I'm a bit surprised that its "Environments" documentation [2] differs from the previous in that it doesn't list "User-space task".  Is that accurate?
> 

Documentation issue, the clause applies to both calls indeed.

> 
> [1] http://www.xenomai.org/documentation/xenomai-2.5/html/api/group__pipe.html#g62cb64807c2c843f8e8eebb2dc3a7d4e
> [2] http://www.xenomai.org/documentation/xenomai-2.5/html/api/group__pipe.html#g731e5ef18007dcd58a9346bea66abbc6
> 
> Thanks,
> -Steve
> 
> 


-- 
Philippe.