linux-kernel - Re: x86 and I/O APIC IRQ domains

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Date:	Wed, 1 Aug 2012 16:39:47 +0200
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: x86 and I/O APIC IRQ domains

On Wed, Aug 01, 2012 at 03:58:39PM +0200, Thierry Reding wrote:
> Hi,
> 
> I've been working on an x86 platform and want to use DT. However I've
> hit a snag when trying to instantiate the I/O APIC. I've been trying to
> follow what the CE4100 does and most things seem to work fine but when
> I add the DT node for the I/O APIC things start to fail. I've been able
> to trace the issue to x86_add_irq_domains(), which in turn calls
> ioapic_add_ofnode() from which irq_domain_add_legacy() is called.
> 
> The platform that I use hits the WARN_ON(!irq_data || irq_data->domain).
> Looking further this seems to be caused by all irq_get_irq_data(irq)
> returning NULL for irq >= 16. That in turn I think is due to
> init_ISA_irqs() setting up only the first NR_IRQS_LEGACY interrupts.
> However the call to irq_domain_add_legacy() wants 32 interrupts.

So it turns out that this is not due to init_ISA_irqs(). However,
increasing NR_IRQS_LEGACY to 32 allows the code to get past that.

> This was introduced by commit b4e5185 "irq_domain/x86: Convert x86
> (embedded) to use common irq_domain)". I wonder what I'm doing wrong. I
> don't get how this is made to work on CE4100.
> 
> Later the code crashes, but I can't exactly pinpoint the location
> because the oops doesn't fit on the screen. I don't have a serial port
> that I can use instead, so is there anything else I can do to obtain a
> complete backtrace?

This crash is in irq_domain_add_legacy() which tries to call the IRQ
domain's .map() function, which isn't implemented for the I/O APIC.
Adding an empty function allows the system to boot without any
noticeable errors.

I need to read up on the whole I/O APIC stuff because this is not really
a fix and I don't know any better. Also, the CE4100 should exhibit the
same issue after the above-mentioned commit. Can somebody more
knowledgeable shed some light on this?

Thierry

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