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

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Message-ID: <20071107151837.3cbe3a67@the-village.bc.nu>
Date:	Wed, 7 Nov 2007 15:18:37 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	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.

> > 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.

No it is not. The platform IRQ code is responsible for ensuring that 0 is
not a real IRQ and doing any neccessary remapping.

Large parts of the kernel assume that
	-	IRQ 0 is "no IRQ assigned" (serial, pci, ide etc )
	-	IRQ is *unsigned*

> We do have some devices that are physically on IRQ#0 that otherwise work
> fine, they aren't ATA devices mind you, but to claim that IRQ#0 isn't a
> valid vector is not in line with what hardware actually does, whether
> it's a good idea or not. In our case the IRQ vector maps to an exception
> offset, which we bump down to zero. We could force an off-by-1 there so
> that the math that indexes IRQ#0 is bumped up one, but that entails
> fixing up every one of our IRQ numbers for no obvious gain.
> 
> I don't really see any value in purposely crippling the range of
> allowable vectors for these machines. Though I don't mind switching to a
> NO_IRQ comparison instead of the < 0 case, so both can be handled.

NO_IRQ is an obsolete old-IDE hack.

http://www.uwsg.indiana.edu/hypermail/linux/kernel/0511.2/2197.html

http://www.uwsg.indiana.edu/hypermail/linux/kernel/0511.2/1789.html

Alan
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