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Date:	Wed, 20 Mar 2013 13:23:13 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Jiri Kosina <jkosina@...e.cz>
cc:	Daniel Vetter <daniel.vetter@...ll.ch>,
	Chris Wilson <chris@...is-wilson.co.uk>,
	Greg KH <greg@...ah.com>,
	Harald Arnesen <skogtun.linux@...il.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Peter Hurley <peter@...leysoftware.com>,
	Thomas Meyer <thomas@...3r.de>,
	Shawn Starr <shawn.starr@...ers.com>,
	USB list <linux-usb@...r.kernel.org>,
	<linux-acpi@...r.kernel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	<linux-pci@...r.kernel.org>, Yinghai Lu <yinghai@...nel.org>,
	Imre Deak <imre.deak@...el.com>,
	Daniel Kurtz <djkurtz@...omium.org>,
	<dri-devel@...ts.freedesktop.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
	Arkadiusz Miskiewicz <a.miskiewicz@...il.com>
Subject: Re: gm45 intel gfx can generate non-MSI irq# in MSI mode (was Re:
 [PATCH] drm/i915: stop using GMBUS IRQs on Gen4 chips (was Re: [3.9-rc1]
 irq 16: nobody cared (was [3.9-rc1] very poor interrupt respo

On Wed, 20 Mar 2013, Jiri Kosina wrote:

> > I don't know of any way.  In fact, I have been thinking of writing a 
> > test driver module, with a module parameter telling it which IRQ number 
> > to register for.  It seems like the sort of thing that would be useful 
> > to have, from time to time.
> 
> Ok, so how about this?
> Daniel, is it enough to make the problem appear on your system (by 
> building this into the kernel and booting with dummy-irq.irq=16)?
> 
> Thanks.
> 
> 
> 
> 
> 
> From: Jiri Kosina <jkosina@...e.cz>
> Subject: [PATCH] dummy-irq: introduce a dummy IRQ handler driver
> 
> This module accepts a single 'irq' parameter, which it should register for.
> 
> Its sole purpose is to help with debugging of IRQ sharing problems, by
> force-enabling IRQ that would otherwise be disabled.
> 
> Suggested-by: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Jiri Kosina <jkosina@...e.cz>

This is just what I was thinking of.  Three extremely minor 
suggestions...

> +static irqreturn_t dummy_interrupt(int irq, void *dev_id)
> +{
> +	static int count = 0;
> +
> +	if (count == 0) {
> +		printk("dummy-irq: interrupt occured on IRQ %d\n", irq);

You probably should put a severity level here.  KERN_INFO?

> +		count++;
> +	}
> +
> +	return IRQ_NONE;
> +}
> +
> +static int __init dummy_irq_init(void)
> +{
> +	if (request_irq(irq, &dummy_interrupt, IRQF_SHARED, "dummy_irq", &irq)) {
> +		printk(KERN_ERR "dummy-irq: cannot register IRQ %d\n", irq);
> +		return -EIO;
> +	}
> +	printk(KERN_INFO "dummy-irq: registered for IRQ %d\n", irq);
> +	return 0;
> +}
> +
> +static void __exit dummy_irq_exit(void)
> +{
> +	printk(KERN_INFO "dummy-irq unloaded\n");
> +	free_irq(irq, &irq);
> +	return;

A return statement isn't needed here.

> +}
> +
> +module_init(dummy_irq_init);
> +module_exit(dummy_irq_exit);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Jiri Kosina");
> +module_param_named(irq, irq, uint, 0444);

module_param is good enough when the parameter's name is the same as 
the variable's name.

> +MODULE_PARM_DESC(irq, "The IRQ to register for");

Alan Stern

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