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Date: Mon, 30 Nov 2009 22:15:38 +0200
Message-ID: <cd73a99e0911301215i7a61b468w3b49325bd1fda3ac@mail.gmail.com>
Subject: Re: Get rid of IRQF_DISABLED - (was [PATCH] genirq: warn about 
	IRQF_SHARED|IRQF_DISABLED)
From: Andrew Victor <avictor.za@gmail.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>,
       David Brownell <dbrownell@users.sourceforge.net>,
       Eric Miao <eric.y.miao@gmail.com>,
       Peter Zijlstra <peterz@infradead.org>,
       John Stultz <johnstul@us.ibm.com>,
       Rusty Russell <rusty@rustcorp.com.au>,
       Jamie Lokier <jamie@shareable.org>, LKML <linux-kernel@vger.kernel.org>,
       Remy Bohmer <linux@bohmer.net>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Hugh Dickins <hugh.dickins@tiscali.co.uk>,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Ingo Molnar <mingo@elte.hu>, Andrea Gallo <andrea.gallo@stericsson.com>,
       =?ISO-8859-1?Q?Uwe_Kleine=2DK=F6nig?= 
	<u.kleine-koenig@pengutronix.de>,
       Thomas Gleixner <tglx@linutronix.de>, Nicolas Pitre <nico@marvell.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       linux-arm-kernel@lists.infradead.org
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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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