linux-kernel - Re: [RFC] killing the NR

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Message-Id: <1171665190.5644.96.camel@localhost.localdomain>
Date:	Sat, 17 Feb 2007 09:33:10 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [RFC] killing the NR_IRQS arrays.

On Fri, 2007-02-16 at 13:41 +0100, Ingo Molnar wrote:
> * Eric W. Biederman <ebiederm@...ssion.com> wrote:
> 
> > So I propose we remove all assumptions from the code that we actually 
> > have an array of irqs.  That will allow for irq_desc to be dynamically 
> > allocated instead of statically allocated saving memory and reducing 
> > kernel complexity.
> 
> hm. I'd suggest to do this without changing request_irq() - and then we 
> could avoid the 'massive, every driver affected' change, right?
> 
> i.e. because we'll (have to) have an nr_to_desc() and desc_to_nr() 
> mapping facility anyway, lets just not change the driver APIs massively. 
> There dont seem to be that many drivers that assume that irq_desc[] is 
> an array - are there?
> 
> otherwise, in terms of the irqchips infrastructure and the API between 
> genirq and the irqchip arch-level drivers, this change makes quite a bit 
> of sense i think.
> 
> or am i missing something fundamental?

Well, I don't want to see anything like desc_to_nr / nr_to_desc unless
the number in question is a virtual number. That is, there is no way we
should go that way and keep passing a HW number through request_irq.
That would just be a total nightmare for powerpc and sparc at least.

What we can do is generalize the powerpc virtual irq scheme though. You
can see the implementation in arch/powerpc/kernel/irq.c starting from
the definition of irq_alloc_host() though for some stupid reason, I've
put all the documentation in include/asm-powerpc/irq.h so you might want
to start there.

Once the IRQ numbers are virtualized, it becomes easier to slowly
migrate things to use irq_desc_t * while still having a virutal number
available.

Once everything has been migrated, we can then get rid of the virtual
numbers completely except maybe for an optional 16 entries array for
legacy cruft.

Ben.

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