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

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Message-ID: <20071129102506.GB22793@flint.arm.linux.org.uk>
Date:	Thu, 29 Nov 2007 10:25:06 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Remy Bohmer <linux@...mer.net>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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, Nov 29, 2007 at 11:14:30AM +0100, Remy Bohmer wrote:
> I do not think Russell is right here with assuming that the wrong
> interrupt handler type is used. Looking at the behaviour of the
> mainline kernel (non-RT), the implementation is quite different: On
> mainline the handle_simple_irq() in chip.c is not re-entrant, the
> masking is **only** done in case of errors, and therefor never
> unmasked again, of course.

The issue comes down to a very simple question:

  Are you using handle_simple_irq() with an interrupt which can be masked?

If the answer to that question is 'yes' then you're using the wrong handler.
If 'no' then it's the right handler and the mask/unmask methods associated
with the interrupt will be no-ops.

Therefore, if you have mask/unmask methods for a simple IRQ which actually
do something, clearly the first answer is the one which applies.  You're
using the wrong handler.  Use the level or edge handlers instead.
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