linux-kernel - Re: [PATCH 1/2] libata: Support PIO polling-only hosts.

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Date:	Wed, 7 Nov 2007 23:53:26 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Mark Lord <lkml@....ca>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Jeff Garzik <jeff@...zik.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxsh-dev@...ts.sourceforge.net
Subject: Re: [PATCH 1/2] libata: Support PIO polling-only hosts.

On Wed, Nov 07, 2007 at 09:09:30AM -0500, Mark Lord wrote:
> Paul Mundt wrote:
> >On Wed, Nov 07, 2007 at 01:09:40PM +0000, Alan Cox wrote:
> >>On Wed, 7 Nov 2007 17:10:52 +0900
> >>Paul Mundt <lethal@...ux-sh.org> wrote:
> >>>By default ata_host_activate() expects a valid IRQ in order to
> >>>successfully register the host. This patch enables a special case
> >>>for registering polling-only hosts that either don't have IRQs
> >>>or have buggy IRQ generation (either in terms of handling or
> >>>sensing), which otherwise work fine.
> >>>
> >>>Hosts that want to use polling mode can simply set ATA_FLAG_PIO_POLLING
> >>>and pass in a NULL IRQ handler or invalid (< 0) IRQ.
> >>NAK
> >>
> >>Zero is "no IRQ", please use that for polling not "< 0"
> >>
> >However, platform_get_irq() will happily return IRQ#0, and it's a valid
> >vector on plenty of machines. NO_IRQ is also < 0 on at least FR-V, ARM,
> >blackin, PA-RISC, some PowerPC, and even IDE.
> 
> Too bad.  The Penultimate Penguin wants zero to continue to mean "no IRQ".
> 
> Dig into the archives for multiple threads on this exact topic.
> The end result is that "0" means "no IRQ".  If your physical IRQ actually
> is the number 0, then reencode it to some other value for this purpose.
> 
I've read the threads, but this does little to do with the fact it's
still a perfectly valid vector, and I'm not about to force every IRQ
vector on my platform off-by-1 in order to satisfy a religious point of
view with zero reflection on what the hardware actually looks like.

So I'll change the check to IRQ#0 == invalid, but if that's to be
enforced kernel-wide, then all of the existing NO_IRQ cases should be
ripped out and set to 0. This way at least people are getting screwed
consistently, rather than just in particular subsystems.

> Yes, a bit of pain, but that's how many parts of the kernel expect it,

Just as many parts of the kernel make no such assumption.

> and in the end it's no more overall hassle than doing it differently might
> have been.
> 
Spoken like someone who doesn't have to contend with off-by-1 IRQ
vectors as a result of an entirely cosmetic change. It's certainly easier
to parrot a party line when you aren't being bitten by it.

So again, I'll make the change, but it's utter nonsense.
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