Philippe Gerum authored Jan 17, 2021 and Jan Kiszka committed Jan 19, 2021

This patch starts a series aiming at dropping the (functionally
redundant) APC interface entirely.

APCs are a relic from the Dark Ages, with no upside compared to open
coded requests triggering virtual/synthetic IRQs to be handled by the
root domain. As a matter of fact, an APC does run as a client handler
of a synthetic IRQ under the hood.

With this change, all APCs will not be multiplexed over a single
synthetic IRQ anymore but each deferred procedure is going to be
assigned its own synthetic IRQ channel, which is hardly a problem
since we only have a couple of APCs to deal with, much fewer than the
number of synthetic IRQs available to us.

As a result, /proc/xenomai/apc will not be available for inspecting
the trigger count of synthetic interrupts used by the core
anymore. Since the I-pipe is on its way out, having this obscure
feature dropped in this context seems acceptable. However, the related
information will still be available to Dovetail-based builds directly
from /proc/interrupts, as synthetic IRQs are (mostly) regular
interrupts there.

Start with using a virtual/synthetic IRQ for kicking the wakeup
procedure for message pipes, dropping the dependency on APCs.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>