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

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Date:	Wed, 14 Jan 2009 17:56:00 -0500
From:	Jon Masters <jcm@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Ingo Molnar <mingo@...e.hu>, 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

On Wed, 2009-01-14 at 14:42 -0800, Eric W. Biederman wrote:
> Jon Masters <jcm@...hat.com> writes:
> 
> > On Wed, 2009-01-14 at 12:40 +0100, Ingo Molnar wrote:
> >
> >> it's not just -rt, but it is also needed for the concept of threaded IRQ 
> >> handlers - which was discussed at the Kernel Summit to be desired for 
> >> mainline.
> >
> > Right. I'm poking at Thomas' patches and hope to post something soon on
> > that front - I'm acutely aware that this will be impacted aswell but
> > because it's vaguely RT related had banded it under that banner.
> 
> Stepping back a moment.  The only way I can see this working reliably
> is if we disable the boot interrupt.  Anything that leaves the boot interrupt
> enabled means that when we disable the primary interrupt the boot interrupt
> will scream, and thus we must disable it as well.
> 
> Which leads to my problem with the entire development process of this feature.
> 
>  People want the feature.
>  People don't want to pay attention to the limits of the hardware.
>  Which leads to countless broken patches proposed.

Is a patch broken because hardware has limitations? If that were always
true then many of the patches we see in the kernel wouldn't be there.

> Which leads me to conclude.
> - IRQ handling in the RT kernel is hopelessly broken.

Nope. It's done in a very similar way to other real time kernels already
out there - really there are only so many ways to do this.

> - IRQ threads are a bad idea.

Why? IRQ threads actually make life so much easier - you have a task
context, you can do everything inside that rather than scheduling all
kinds of deferred work (that in RT will be done in another task later),
and so forth.

> None of this works reliably on level triggered ioapic irqs.

Level triggered IOAPIC IRQs have quirks, film at 11!

Jon.


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