linux-kernel - Re: [PATCH PREEMPT_RT]: On AT91 ARM: GPIO Interrupt handling can/will stall forever

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Message-ID: <Pine.LNX.4.58.0711290849140.21439@gandalf.stny.rr.com>
Date:	Thu, 29 Nov 2007 08:57:01 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Remy Bohmer <linux@...mer.net>, Daniel Walker <dwalker@...sta.com>,
	RT <linux-rt-users@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	ARM Linux Mailing List 
	<linux-arm-kernel@...ts.arm.linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH PREEMPT_RT]: On AT91 ARM: GPIO Interrupt handling can/will
 stall forever

On Thu, 29 Nov 2007, Russell King - ARM Linux wrote:

> On Thu, Nov 29, 2007 at 12:27:30PM +0100, Remy Bohmer wrote:
>
> Ah, and looking at the changes to the file, the addition of the mask
> and unmask was done by someone who didn't understand what this was
> trying to do.  So that change should be backed out.
>

Perhaps only part of the change should be backed out.  The part that masks
the irq in the handle_simple_irq code.

That's from commit 76d2160147f43f982dfe881404cfde9fd0a9da21 which is to
not disable an irq line when disable_irq is called.  A form of lazy
disable irq.

This speeds up code that uses disable_irq, since the line is only masked
when an interrupt actually arrives. Using disable_irq / enable_irq does no
IRQ chip modifications if an interrupt from the IRQ line does not arrive
between the two.

Now the question is, can something that uses handle_simple_irq call
disable_irq?  If there is no mask function, I would assume that this would
be a noop in such a case. If this is true, then we could remove the mask
from handle_simple_irq. But then we might want to add a BUG() in
disable_irq for simple_irqs.

-- Steve

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