linux-kernel - Re: [patch 46/47] powerpc: Use new irq allocator

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Date:	Mon, 4 Oct 2010 10:46:00 -0600
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Russell King <linux@....linux.org.uk>,
	David Woodhouse <dwmw2@...radead.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: [patch 46/47] powerpc: Use new irq allocator

On Mon, Oct 04, 2010 at 09:54:19AM +1100, Benjamin Herrenschmidt wrote:
> On Sun, 2010-10-03 at 09:53 -0700, Eric W. Biederman wrote:
> > Thomas Gleixner <tglx@...utronix.de> writes:
> > 
> > >> That would make things much cleaner and in fact move one large step
> > >> toward being able to make powerpc virq scheme generic, which seems to be
> > >> a good idea from what I've heard :-)
> > >
> > > Yep.
> > 
> > I'm not certain about making the ppc virq scheme generic.  Maybe it is
> > just my distorted impression but I have the understanding that ppc irq
> > numbers mean nothing and are totally unstable whereas on x86 irq numbers
> > in general are stable (across kernel upgrades and changes in device
> > probe order) and the irq number has a useful hardware meaning.  Which
> > means you don't have to go through several layers of translation tables
> > to figure out which hardware pin you are talking about.
[...]
>
> The main deal I'd say is that in embedded land (and to some extent I
> suspect that's going to happen more with x86), you quickly end up with
> multiple interrupt domains, via cascaded controllers of all kinds etc...
[...]
>  - Appart from the risk of breaking crap that parses /proc/interrupts,
> adding the HW irq information there would be trivial and solve your
> problem.
> 
> So overall, I don't see a problem at all. And it makes handling of
> arbitrary combinations of interrupt domains (cascaded PICs) very very
> easy indeed.

Right, the stability of irq numbers is more of a user-interface
problem than a technical deficiency with virq, and a solvable one at
that.  If fact, I'd go as far as saying that having irq numbers mean
nothing is a *virtue* of the approach because it forces developers to
look at the real irq controller attachment rather than some
hwirq==linux-irq assumption.

As mentioned in my other reply, I'd be perfectly happy to stop exposing
linux irq numbers to userspace entirely, but I realize that would
break /proc/interrupts users.  However, I do completely agree that
exposing the hwirq information is very much required.

g.
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