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

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Date:	Wed, 11 Jan 2012 02:15:11 -0800
From:	Andres Salomon <dilinger@...ued.net>
To:	Jens Rottmann <JRottmann@...PERTEmbedded.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	linux-geode@...ts.infradead.org
Subject: Re: [PATCH] cs5535-clockevt: allow the MFGPT IRQ to be shared

Seems fine to me

Acked-by: Andres Salomon <dilinger@...ued.net>

On Thu, 22 Dec 2011 17:35:50 +0100
Jens Rottmann <JRottmann@...PERTEmbedded.de> wrote:

> 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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