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

Jiang Liu
2014-06-12T12:40:09Z

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
>