linux-kernel - Re: [PATCH 0/7] Boot IRQ quirks and rerouting

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Date:	Wed, 4 Jun 2008 18:08:49 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
cc:	Stefan Assmann <sassmann@...e.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Olaf Dabrunz <od@...e.de>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jon Masters <jonathan@...masters.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] Boot IRQ quirks and rerouting

On Wed, 4 Jun 2008, Maciej W. Rozycki wrote:
> On Wed, 4 Jun 2008, Stefan Assmann wrote:
>  What's the reasoning behind the option in this case?  As I understand 
> there are two cases possible:
> 
> 1. Secondary, etc. I/O APIC inputs are not masked under any circumstances.  
>    No legacy INTx redirection happens, nothing to be done.
> 
> 2. Secondary, etc. I/O APIC inputs are to be masked from time to time.  
>    That would cause legacy INTx redirection for the affected chipsets in 
>    situations where an interrupt arrives at a masked I/O APIC input.  This 
>    interrupt is delivered to an input of the primary I/O APIC which cannot 
>    be masked because of other devices wired to it.
> 
>    OK -- that means the interrupt is delivered anyway (and perhaps 
>    discarded in the handler, but that does not matter here), so why to do

It does matter. When the interrupt is _not_ handled then it comes back
immediately for ever and after a while the kernel decides to disable
the legacy int, because nobody cares about the interrupt.

What happens is:

     irq_disable(secondary);
     
     irq line of the secondary ioapic becomes active

     legacy irq happens

repeat:
     legacy irq disable
     handler runs and discards
     legacy irq enable

     legacy interrupt is still active due to rerouting
     if (count < max)
        goto repeat
     else
	disable legacy irq forever

Thanks,
	tglx
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