linux-kernel - Re: [PATCH] serial: Use real irq on UART0 (IRQ = 0) on PPC4xx systems

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Date:	Mon, 20 Nov 2006 22:21:27 +0100 (CET)
From:	Guennadi Liakhovetski <g.liakhovetski@....de>
To:	Russell King <rmk+lkml@....linux.org.uk>
cc:	Alan <alan@...rguk.ukuu.org.uk>, Stefan Roese <ml@...fan-roese.de>,
	linux-kernel@...r.kernel.org, linuxppc-embedded@...abs.org
Subject: Re: [PATCH] serial: Use real irq on UART0 (IRQ = 0) on PPC4xx systems

On Mon, 20 Nov 2006, Russell King wrote:

> > On Mon, 20 Nov 2006 12:54:32 +0100 (MET)
> > Stefan Roese <ml@...fan-roese.de> wrote:
> > > Let's see, if I got this right. You mean that on such a platform, where 0 is a 
> > > valid physical IRQ, we should assign another value as virtual IRQ number (not 
> > > 0 and not -1 of course). And then the platform "pic" implementation should 
> > > take care of the remapping of these virtual IRQ numbers to the physical 
> > > numbers.
> 
> Since IRQ0 is not valid, can we arrange for the generic interrupt
> infrastructure to always fail it's allocation, and then remove the
> utterly unused bloatful irq_desc[0] ?
> 
> Didn't think so since x86 folk would scream.  Wait a moment, x86 can
> map IRQ0 to some other number for the timer interrupt, just like
> other architectures are being forced to map their UART interrupts.

I think, what Russell means, is this:

#define is_real_interrupt(irq) ((irq) != NO_IRQ)

where the NO_IRQ macro has been introduced a LONG time ago specifically 
for this purpose, and is conveniently defined on some platforms to 
(unsigned int)-1 or similar, including asm-powerpc/irq.h. And yes, this 
has been discussed MANY times.

Thanks
Guennadi
---
Guennadi Liakhovetski
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