On Wed, Oct 12, 2022, at 7:22 PM, Linus Torvalds wrote:

The NO_IRQ thing is mainly actually defined by a few drivers that just
never got converted to the proper world order, and even then you can
see the confusion (ie some drivers use "-1", others use "0", and yet
others use "((unsigned int)(-1)".

The last time I looked at removing it for arch/arm/, one problem was
that there were a number of platforms using IRQ 0 as a valid number.
We have converted most of them in the meantime, leaving now only
mach-rpc and mach-footbridge. For the other platforms, we just
renumbered all interrupts to add one, but footbridge apparently
relies on hardcoded ISA interrupts in device drivers. For rpc,
it looks like IRQ 0 (printer) already wouldn't work, and it
looks like there was never a driver referencing it either.

I see that openrisc and parisc also still define NO_IRQ to -1, but at
least openrisc already relies on 0 being the invalid IRQ (from
CONFIG_IRQ_DOMAIN), probably parisc as well.

     Arnd