linux-kernel - [PATCH] cs5535-clockevt: allow the MFGPT IRQ to be shared

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Date:	Thu, 22 Dec 2011 17:35:50 +0100
From:	Jens Rottmann <JRottmann@...PERTEmbedded.de>
To:	Andres Salomon <dilinger@...ued.net>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel@...r.kernel.org, linux-geode@...ts.infradead.org
Subject: [PATCH] cs5535-clockevt: allow the MFGPT IRQ to be shared

cs5535-clockevt: allow the MFGPT IRQ to be shared

Shared timer IRQs are not a good solution, however the Geode platform has no
APIC, IRQs are a scarce resource and there is no technical reason to forbid it
rightaway.  Increased latencies and overhead due to sharing are still better
than a driver refusing to load.

Signed-off-by: Jens Rottmann <JRottmann@...PERTEmbedded.de>
---

Hi,

I tested this a bit longer, this time with MFGPT IRQ actually being triggered,
with cs5535-clockevt driver loaded first or second when sharing the IRQ, with
some CPU load or without.

I didn't encounter any negative effects of this change. I did have suspend
problems with cs5535-clockevt, but that was in no way different from before I
applied this patch, it's an unrelated BIOS issue.

Thomas Gleixner wrote:
> No, you can share a timer irq. The other drivers don't have the SHARED
> flag set because they are on exclusive irq lines, ...

Is this an ACK?

> shared irqs suck and you figure that out once you try to
> use that shared timer irq on a preempt-rt enabled kernel.

Or a NACK?  :-|   (I did address this in the commit log.)

The kernel I tested with was 3.2-rc6, CONFIG_SMP=y and CONFIG_PREEMPT=y. As I
said, I didn't notice anything bad happening.

Thanks and have a nice christmas holiday,
Jens

--- linux-3.2-rc6/drivers/clocksource/cs5535-clockevt.c
+++ allow_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,
 };

_

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