linux-kernel - Re: PCI, ACPI, IRQ, IOAPIC: reroute PCI interrupt to legacy boot interrupt equivalent

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Date:	Tue, 13 Jan 2009 02:47:23 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jon Masters <jcm@...hat.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Bjorn Helgaas <bjorn.helgaas@...com>,
	Stefan Assmann <sassmann@...e.de>, Len Brown <lenb@...nel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Olaf Dabrunz <od@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-acpi@...r.kernel.org, Sven Dietrich <sdietrich@...ell.com>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>
Subject: Re: PCI, ACPI, IRQ, IOAPIC: reroute PCI interrupt to legacy boot
	interrupt equivalent


* Jon Masters <jcm@...hat.com> wrote:

> On Mon, 2009-01-12 at 15:36 -0800, Eric W. Biederman wrote:
> 
> > This hardware behavior is not specific to boot interrupts or Intel.
> 
> It's not specific to Intel, but it is a specific compatibility behavior.
> 
> > Is this case really so interesting and compelling that we want to 
> > fight through and figure what we need to do to make this work reliably 
> > on every x86 chipset?
> 
> How else do you propose implementing IRQ handling in e.g. the RT kernel? 
> We get a hardware interrupt, we can't FastEOI, we can't process 
> synchronously, we can't do all of those things you might expect. 
> Implementing RT requires that we delay handling of the IRQ until 
> arbitrarily later in the future when we get around to it.

a number of mainline drivers also mask/unmask irqs from within the IRQ 
handler. It's not particularly smart in a native driver, but can happen - 
and if we get an active line after that point (and this can happen because 
the driver is active), we are in trouble.

	Ingo
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