linux-kernel - Re: [PATCH v2] irqchip: omap-intc: add support for spurious irq handling

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Date:	Thu, 10 Dec 2015 20:46:50 +0530
From:	Sekhar Nori <nsekhar@...com>
To:	Felipe Balbi <balbi@...com>, Tony Lindgren <tony@...mide.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <marc.zyngier@....com>
CC:	John Ogness <john.ogness@...utronix.de>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] irqchip: omap-intc: add support for spurious irq
 handling

Hi Felipe,

On Tuesday 08 December 2015 07:15 PM, Felipe Balbi wrote:
> Sekhar Nori <nsekhar@...com> writes:

>> +	/*
>> +	 * A spurious IRQ can result if interrupt that triggered the
>> +	 * sorting is no longer active during the sorting (10 INTC
>> +	 * functional clock cycles after interrupt assertion). Or a
>> +	 * change in interrupt mask affected the result during sorting
>> +	 * time. There is no special handling required except ignoring
>> +	 * the SIR register value just read and retrying.
>> +	 * See section 6.2.5 of AM335x TRM Literature Number: SPRUH73K
>> +	 *
>> +	 * Many a times, a spurious interrupt situation has been fixed
>> +	 * by adding a flush for the posted write acking the IRQ in
>> +	 * the device driver. Typically, this is going be the device
>> +	 * driver whose interrupt was handled just before the spurious
>> +	 * IRQ occurred. Pay attention to those device drivers if you
>> +	 * run into hitting the spurious IRQ condition below.
>> +	 */
>> +	if ((irqnr & SPURIOUSIRQ_MASK) == SPURIOUSIRQ_MASK) {
> 
> sounds like unlikely() wouldn't hurt here.

I can add, but looks like it does not make a big difference. See below.

> 
>> +		pr_err_once("%s: spurious irq!\n", __func__);
>> +		irq_err_count++;
>> +		omap_ack_irq(NULL);
>> +		return;
>> +	}
>> +
>>  	irqnr &= ACTIVEIRQ_MASK;
>> -	WARN_ONCE(!irqnr, "Spurious IRQ ?\n");
>>  	handle_domain_irq(domain, irqnr, regs);
> 
> care to run kernel function profiler against omap_intc_handle_irq()
> before and after this patch ?

Before this patch I see average running time time of 34us. That
increases to 37.8us after this patch. With unlikely() the number I got
was 37.4us. So the benefit with unlikely() is in the noise range.

This was using AM335x EVM at 720 MHz.

Thanks,
Sekhar
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