Re: [PATCH V2] acpi, pci, irq: account for early penalty assignment

From: Rafael J. Wysocki
Date: Tue Mar 08 2016 - 16:00:25 EST

On Tue, Mar 8, 2016 at 8:04 PM, Sinan Kaya <okaya@xxxxxxxxxxxxxx> wrote:
>>>>> I think there are two issues here that should be teased apart a bit
>>>>> more:
>>>>> 1) Trigger settings: If the IRQ is configured as anything other than
>>>>> level-triggered, active-low, we can't use it at all for a PCI
>>>>> interrupt, and we should return an "infinite" penalty. We currently
>>>>> increase the penalty for the SCI IRQ if it's not level/low, but
>>>>> doesn't it apply to *all* IRQs, not just the SCI IRQ?
>>>> It makes sense for SCI as it is Intel specific.
>>>> Unfortunately, this cannot be done in an arch independent way. Of course,
>>>> ARM had to implement its own thing. While level-triggered, active-low is
>>>> good for intel world, it is not for the ARM world. ARM uses active-high
>>>> level triggered.
>>> I'm confused. I don't think SCI is Intel-specific. Per PCI Spec
>>> r3.0, sec 2.2.6, PCI interrupts are level-sensitive, asserted low.
>>> Per ACPI Spec v3.0, sec 2.1, the SCI is an "active, low, shareable,
>>> level interrupt".

That's correct.

The SCI isn't Intel-specific or even x86-specific for that matter.
However, it is not available on hardware-reduced ACPI platforms which
are all ARM using ACPI.

>>> Are you saying SCI is active-high on ARM? If so, I don't think that's
>>> necessarily a huge problem, although we'd have to audit the ACPI code
>>> to make sure we handle it correctly.
> We don't have an SCI interrupt on ARM. That's why, I assumed it is an Intel specific
> interrupt. However, all legacy interrupts are active-high level sensitive. This is a
> limitation of the ARM GIC Interrupt Controller.
> Here is what a PCI Link object looks like.
> Device(LN0D){
> Name(_HID, EISAID("PNP0C0F")) // PCI interrupt link
> Name(_UID, 4)
> Name(_PRS, ResourceTemplate(){
> Interrupt(ResourceProducer, Level, ActiveHigh, Exclusive, , ,) {123}
> })
> Method(_DIS) {}
> Method(_CRS) { Return (_PRS) }
> Method(_SRS, 1) {}
> }
>>> The point here is that a PCI Interrupt Link can only use an IRQ that
>>> is level-triggered, active low. If an IRQ is already set to any other
>>> state, whether for an ISA device or for an active-high SCI, we can't
>>> use it for a PCI Interrupt Link.
> Unfortunately, this still doesn't hold.

I think that the original active-low requirement is related to the
fact that those IRQ can be shared. If they aren't shared, I guess the
point is slightly moot.