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 22:15:38 +0200
From:	Andrew Victor <avictor.za@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Eric Miao <eric.y.miao@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <johnstul@...ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jamie Lokier <jamie@...reable.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Remy Bohmer <linux@...mer.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrea Gallo <andrea.gallo@...ricsson.com>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nicolas Pitre <nico@...vell.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Get rid of IRQF_DISABLED - (was [PATCH] genirq: warn about 
	IRQF_SHARED|IRQF_DISABLED)

hi,

> 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 AT91 case I don't think this shouldn't matter.

The AT91's have a priority-level interrupt controller, so:
  1. a lower-priority interrupt won't interrupt a higher-priority
  2. shared interrupts cannot interrupt each other until irq_finish()
is called (a write to AIC_EOICR)

Since the Timer, DBGU serial port (and other system peripherals) are
on the same priority level they cannot interrupt each other.
(ie, basically as-if always irqs-disabled).

The case of irqs-enabled does means that a higher-priority interrupt
could interrupt [*], but it's not a shared-IRQ in that case.

([*] The system peripherals have the highest priority by default, so
the user would need to override the defaults for this to occur)


Regards,
 Andrew Victor
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