linux-kernel - Re: IRQF_TIMER

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Date:	Wed, 14 Dec 2011 19:37:39 +0100
From:	Jens Rottmann <JRottmann@...PERTEmbedded.de>
To:	Andres Salomon <dilinger@...ued.net>
CC:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	linux-geode@...ts.infradead.org
Subject: Re: IRQF_TIMER | IRQF_SHARED ?

Jens Rottmann schrieb:
> Andres Salomon schrieb:
>> Just loading cs5535-clockevt should start the periodic timer.
>> On my XO-1, IRQ 7 starts firing immediately.
> Hmm, no, doesn't work. :-|  In /proc/interrupts IRQ 0 gets increased,
> not (in my case) IRQ 11.

Update: I found that SMP-enabled kernels (like the generic one I was using) do
load cs5535-clockevt fine but ignore it and keep using the pit timer instead.

Responsible seems kernel/time/tick-common.c: tick_check_new_device()
                /*
                 * If the cpu affinity of the device interrupt can not
                 * be set, ignore it.
                 */
                if (!irq_can_set_affinity(newdev->irq))
                        goto out_bc;

Looks like it has been that way for quite some time, maybe cs5535-clockevt
hasn't ever worked on SMP kernels.

If I turn off SMP, cs5535-clockevt replaces the pit timer and the MFGPT IRQ
starts firing just as you said - at least on 3.0.9. 3.2-rc5 prefers to hang
instead, looks like no timer events get generated. Sigh.

I'll try to narrow it down tomorrow, but now I'm calling it a day.

Cheers
Jens
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