Re: [Xenomai] Interrupt latency close to 1ms on powerpc Xenomai 2.6.4

[Philippe Gerum <rpm@xenomai.org>]

On 10/07/2015 04:25 PM, PRADHAN, MAKARAND (RC-CA) wrote:
> Hi All,
> 
> On further experimentation, I still feel that rt_intr_enable may be causing my user space int handler to jump into secondary domain.
> 

rt_intr_enable() is a secondary domain call, it cannot execute over the
primary domain simply because the infrastructure shared with the regular
kernel that might be involved to enable/disable IRQs requires this. The
nucleus forcibly relaxes the calling thread for this reason. So the
behavior you observe is not a bug, it is actually the intent and
requirement for carrying out such request. Check
ksrc/native/syscall.c:4152, this call is tagged as "lostage", meaning
that it shall run from the lowest pipeline domain stage, i.e. linux.

This is the reason why handling interrupt top-halves in userland is a
bad idea, and also the reason why the RT_INTR support has completely
disappeared from 3.x's Alchemy API (formerly know as 2.x's "native"
API), in favor of a UIO-like model called UDD, standing for "user-space
device driver". There, top-halves must live in kernel space, and
bottom-halves may live in userland, which is the only sane and safe way
to handle interrupts from the latter.

The rationale for this change is stated here:
http://xenomai.org/migrating-from-xenomai-2-x-to-3-x/#irqhandling

-- 
Philippe.