linux-kernel - Re: Get rid of IRQF_DISABLED - (was [PATCH] genirq: warn about IRQF_SHARED|IRQF

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Date:	Mon, 30 Nov 2009 12:53:28 -0800
From:	David Brownell <david-b@...bell.net>
To:	"Russell King - ARM Linux" <linux@....linux.org.uk>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rusty Russell <rusty@...tcorp.com.au>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Eric Miao <eric.y.miao@...il.com>,
	Andrea Gallo <andrea.gallo@...ricsson.com>,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <johnstul@...ibm.com>,
	Nicolas Pitre <nico@...vell.com>,
	Jamie Lokier <jamie@...reable.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Remy Bohmer <linux@...mer.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Uwe Kleine-König" 
	<u.kleine-koenig@...gutronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Get rid of IRQF_DISABLED - (was [PATCH] genirq: warn about IRQF_SHARED|IRQF_DISABLED)

On Monday 30 November 2009, Russell King - ARM Linux wrote:
> On Mon, Nov 30, 2009 at 02:47:02PM +0000, Alan Cox wrote:
> > SHARED|DISABLED ought to WARN_ON() and if that doesn't motivate people
> > then return -EINVAL.
> 
> That is an impossibility.  There is hardware out there (AT91) where
> the timer interrupt is shared with other peripherals, and you end
> up with a mixture of irqs-disabled and irqs-enabled handlers sharing
> the same interrupt.

For the record:  AT91 isn't restricted to the system timers hooked
up on irq 0 ... there's also drivers/clocksource/tcb_clksrc.c (not
at the same hardware priority).

But to concur, this is indeed messy.  Both the system timer and
the serial console generally share the same IRQ; both are very
timing-sensitive.  I've seen console character dropouts after
tweaking timer IRQ handling.  And I've never convinced myself
that Linux handles the hardware IRQ priority on those chips as
well as it could.  


> My point is that if we outlaw irqs-disabled shared interrupts, it puts
> Atmel AT91 support into immediate difficulties.

ISTR that those TCB modules don't share IRQs with other peripherals.

Also, that Linux doesn't use them for much else.  I've yet to see a
three-phase motor driver using the TCB's PWM capabilities, for example.

- Dave
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