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From: Jens Rottmann <JRottmann@LiPPERTEmbedded.de>
To: Andres Salomon <dilinger@queued.net>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, linux-geode@lists.infradead.org
Subject: IRQF_TIMER | IRQF_SHARED ?
Date: Mon, 12 Dec 2011 16:41:25 +0100	[thread overview]
Message-ID: <4EE620A5.7080402@LiPPERTEmbedded.de> (raw)

Hi Andres,

one of our customers tripped over the fact that the MFGPT driver won't share its
IRQ with anyone else. (MFGPT defaulted to same IRQ as audio, MFGPT driver loaded
first, audio fails.) *No big deal!* They don't actually need MFGPT and will
simply disable it. It just made me wonder ...

Why would it be such a bad idea to use IRQF_TIMER | IRQF_SHARED (see patch
below)? mfgpt_tick() already does properly return IRQ_NONE when it feels
unresponsible. I tested it with either driver loaded first and it seemed to work
(well, at least audio worked, don't know how to explicitly test cs5535-clockevt).

I thought about latencies of IRQ sharing being unacceptable for a timer, but ...
- If MFGPT is loaded first there is no additional latency, is there? Audio
  recieves its IRQs only as 2nd in list but that's not a problem.
- If MFGPT is loaded second - well, there is a latency, but without sharing the
  IRQ the driver failed to load at all, so that's still an improvement.

But I did not fail to notice that _none_ of the code in drivers/clocksource/
uses IRQF_SHARED, obviously this must be deliberate.

So, what's so bad about IRQF_TIMER | IRQF_SHARED?

Any education would be welcome, even if combined with flame. :-)

Thanks and best regards,
Jens

--- linux-3.2-rc4/drivers/clocksource/cs5535-clockevt.c
+++ shared_mfgpt_irq/drivers/clocksource/cs5535-clockevt.c
@@ -133,7 +133,7 @@ static irqreturn_t mfgpt_tick(int irq, v

 static struct irqaction mfgptirq  = {
 	.handler = mfgpt_tick,
-	.flags = IRQF_DISABLED | IRQF_NOBALANCING | IRQF_TIMER,
+	.flags = IRQF_DISABLED | IRQF_NOBALANCING | IRQF_TIMER | IRQF_SHARED,
 	.name = DRV_NAME,
 };

_


             reply	other threads:[~2011-12-12 15:56 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-12-12 15:41 Jens Rottmann [this message]
2011-12-12 20:31 ` IRQF_TIMER | IRQF_SHARED ? Andres Salomon
2011-12-12 21:00   ` Martin-Éric Racine
2011-12-12 22:06     ` Andres Salomon
2011-12-12 23:38   ` Thomas Gleixner
2011-12-13 15:49   ` Jens Rottmann
2011-12-14 18:37     ` Jens Rottmann
2011-12-14 18:47       ` Andres Salomon
2011-12-21 15:42         ` [PATCH] cs5535-clockevt: don't ignore MFGPT on SMP-capable kernels Jens Rottmann
2012-01-11 10:10           ` Andres Salomon
2012-01-19 12:57             ` Jens Rottmann
2012-01-23 10:44               ` Andres Salomon
2011-12-21 16:37         ` IRQF_TIMER | IRQF_SHARED ? Jens Rottmann
2011-12-22 16:35         ` [PATCH] cs5535-clockevt: allow the MFGPT IRQ to be shared Jens Rottmann
2012-01-11 10:15           ` Andres Salomon
2012-01-30 13:51         ` [PATCH] cs5535-clockevt: don't ignore MFGPT on SMP-capable kernels Jens Rottmann
2012-01-30 13:59         ` [PATCH] cs5535-clockevt: allow the MFGPT IRQ to be shared Jens Rottmann
2012-02-06  8:20         ` [PATCH resend] cs5535-clockevt: don't ignore MFGPT on SMP-capable kernels Jens Rottmann
2012-02-06  8:23         ` [PATCH resend] cs5535-clockevt: allow the MFGPT IRQ to be shared Jens Rottmann
2011-12-19 14:32       ` "clockevents: Set noop handler in clockevents_exchange_device()" causes hang with cs5535-clockevt Jens Rottmann

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