linux-kernel - Re: [PATCH] ata: Don't use NO_IRQ in pata_of

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Message-ID: <20111205192605.GD29812@localhost.localdomain>
Date:	Mon, 5 Dec 2011 19:26:05 +0000
From:	Dave Martin <dave.martin@...aro.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Anton Vorontsov <cbouatmailru@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	devicetree-discuss@...ts.ozlabs.org,
	LKML <linux-kernel@...r.kernel.org>, linux-ide@...r.kernel.org,
	Randy Dunlap <rdunlap@...otime.net>,
	linux-next@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Jeff Garzik <jgarzik@...hat.com>,
	Pawel Moll <pawel.moll@....com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] ata: Don't use NO_IRQ in pata_of_platform driver

On Mon, Dec 05, 2011 at 01:18:30PM -0500, Nicolas Pitre wrote:
> On Mon, 5 Dec 2011, Dave Martin wrote:
> 
> > On Mon, Dec 05, 2011 at 12:40:16PM -0500, Nicolas Pitre wrote:
> > > On Mon, 5 Dec 2011, Dave Martin wrote:
> > > > On Sat, Dec 03, 2011 at 10:12:53AM +1100, Benjamin Herrenschmidt wrote:
> > > > > On Fri, 2011-12-02 at 11:28 -0800, Linus Torvalds wrote:
> > > > > > Don't *change* NO_IRQ to zero (that whole #define is broken - leave it
> > > > > > around as a marker of brokenness), just start removing it from all the
> > > > > > ARM drivers that use the OF infrastructure. Which is presumably not
> > > > > > all that many yet.
> > > > > > 
> > > > > > So whenever you find breakage, the fix now is to just remove NO_IRQ
> > > > > > tests, and replace them with "!irq".
> > > > > 
> > > > 
> > > > Russell, do you know whether it would make sense to set a timeline for 
> > > > removing NO_IRQ from ARM platforms and migrating to 0 for the no-interrupt
> > > > case?  I'm assuming that this mainly involves migrating existing hard-wired
> > > > code that deals with interrupt numbers to use irq domains.
> > > 
> > > How many drivers do use IRQ #0 to start with?  We might discover that in 
> > > practice there is only a very few cases where this is an issue if 0 
> > > would mean no IRQ.
> > 
> > The total number of files referring to NO_IRQ is not that huge:
> > 
> > arch/arm/	188 matches in 39 files
> > drivers/	174 matches in 84 files
> > 
> > Unfortunately, NO_IRQ is often not spelled "NO_IRQ".  It looks like the assumption
> > "irq < 0 === no irq" may be quite a lot more widespread than "NO_IRQ === no irq".
> > Since there's no specific thing we can grep for (and simply due to volume)
> > finding all such instances may be quite a bit harder.
> [...]
> 
> ARgh.
> 
> My point was about current actual usage of the IRQ numbered 0 which 
> probably prompted the introduction of NO_IRQ in the first place.  What I 
> was saying is that the number of occurrences where IRQ #0 is currently 
> used into drivers that would get confused if 0 would mean no IRQ is 
> probably quite small.

Ah, I misunderstood -- that's a separate issue, but also an important one.
I guess this applies to a fair number of older boards.  One way of fixing
this would be to migrate those boards to use irq domains -- but those boards
may be sporadically maintained.
 
> But as you illustrated, there is a large number of drivers that already 
> assume no IRQ is < 0, even if they don't use any IRQ #0 themselves.  
> That is a much bigger problem to fix.

My concern is that as soon as we start to change this in significant
volume, a _lot_ of stuff is going to break.  Everywhere that an irq value
is passed from one piece of code to another, there is a potential
interface mismatch -- there seems to be no single place where we can
apply a conversion and fix everything.


Cheers
---Dave
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