linux-kernel - Re: oops in ioapic_write

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Date:	Tue, 03 Aug 2010 01:56:32 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Dave Airlie <airlied@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: oops in ioapic_write_entry

Yinghai Lu <yinghai@...nel.org> writes:

> On 08/03/2010 01:00 AM, Eric W. Biederman wrote:
>> Yinghai Lu <yinghai@...nel.org> writes:
>> 
>>>>> Index: linux-2.6/arch/x86/kernel/apic/io_apic.c
>>>>> ===================================================================
>>>>> --- linux-2.6.orig/arch/x86/kernel/apic/io_apic.c
>>>>> +++ linux-2.6/arch/x86/kernel/apic/io_apic.c
>>>>> @@ -1029,10 +1029,7 @@ static int pin_2_irq(int idx, int apic,
>>>>>  	} else {
>>>>>  		u32 gsi = mp_gsi_routing[apic].gsi_base + pin;
>>>>>  
>>>>> -		if (gsi >= NR_IRQS_LEGACY)
>>>>> -			irq = gsi;
>>>>> -		else
>>>>> -			irq = gsi_top + gsi;
>>>>> +		irq = gsi_to_irq(gsi);
>>>>>  	}
>>>>>  
>>>>>  #ifdef CONFIG_X86_32
>>>
>>> what is the point for making irq = gsi_top + gsi when mptable is used instead of acpi?
>> 
>> Because it is only convention that when mptables are used that the
>> first apic pins 0-15 are the ISA irqs.  This thread witnessed and a
>> pci irq that came in pin < 16 that was not an ISA irq.  The truly rare
>> and exotic case would be for the ISA irqs to be outside the first 16
>> ioapic pins but the es7000 did exactly that.
>
> nvidia chipset if acpi is enabled, external pci device will use ioapic from 16 to 23.
>
> if mptable is used, external pci device will not use pin from 16 to 23..., and lot of devices will share same pin.

Exactly.  Pins < 16 are not necessarily ISA irqs, and can be possibly
shared level triggered PCI irqs.  Unfortunately there are strange
boards like the es7000 where pins > 16 are ISA irqs.

The other thing that is gained by having pin_2_irq always remap pins <
16 is we can get away with the numerous hard codes in the arch/x86 and elsewhere
that assume irq < 16 is an ISA irq.

Eric
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