Re: [Patch V4 12/42] x86, ioapic: kill static variable nr_irqs_gsi

[Jiang Liu]

Hi Thomas,
	With all patches applied, we have following code and comments
for this case:
        switch (type) {
        case IOAPIC_DOMAIN_LEGACY:
                /*
                 * Dynamically allocate IRQ number for non-ISA IRQs in
the first 16
                 * GSIs on some weird platforms.
                 */
                if (gsi < nr_legacy_irqs())
                        irq = irq_create_mapping(domain, pin);
                else if (irq_create_strict_mappings(domain, gsi, pin, 1)
== 0)
                        irq = gsi;
                break;


On 2014/6/12 18:58, Thomas Gleixner wrote:
> On Tue, 10 Jun 2014, Jiang Liu wrote:
> 
>> Hi Thomas,
>> 	This piece of code is inherited from current IOAPIC driver
>> and I think it's a workaround for some weird platforms.
>> 	For normal platforms with both 8259A and IOAPIC controllers,
>> legacy ISA IRQs should be connected to both 8259A and IOAPIC pins
>> (ignore timer and cascade IRQs for simplicity). According to comments
>>  in current kernel, there are some platforms on which:
>> 1) some ISA IRQs are only connected to 8259A controllers.
>> 2) the corresponding IOAPIC pins are connected to some non-ISA IRQs.
>> 	For such platforms, IRQ0-15 are used for ISA IRQs and another
>> 16 IRQs just above gsi_top are reserved for IOAPIC pins 0-15 which
>> are connected to non-ISA IRQs.
>> 	I have no real experience with such a platform, but just
>> guessing possible cases according to kernel comments and "Multiple
>> Processor Specification". Please look at these two pictures for quick
>> reference.
>> http://www.manualslib.com/manual/77733/Intel-Multiprocessor.html?page=31#manual
>> http://www.manualslib.com/manual/77733/Intel-Multiprocessor.html?page=63#manual
> 
> Duh. I completely forgot about the 82489 mess.
> 
> We probably want a comment somewhere why we have this gsi + legacy
> thing.
> 
> Thanks,
> 
> 	tglx
> 
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