On Mon, 7 Mar 2016, Bjorn Helgaas wrote:
> On Mon, Mar 07, 2016 at 11:55:43AM -0500, Sinan Kaya wrote:
> > 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".
>
> 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.
>
> 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.
>
> It'd be nice if there were a generic way we could figure out what the
> trigger mode of an IRQ is. I was hoping can_request_irq() was that
> way, but I don't think it is, because it only looks at IRQF_SHARED,
> not at IRQF_TRIGGER_LOW.
>
> Maybe irq_get_trigger_type() is what we want?
Yes, that gives you the trigger typ, if the interrupt is already set up.
> static int pci_compatible_trigger(int irq)
> {
> int type = irq_get_trigger_type(irq);
>
> return (type == IRQ_TYPE_LEVEL_LOW || type == IRQ_TYPE_NONE);
> }
>
> static unsigned int acpi_irq_get_penalty(int irq)
> {
> unsigned int penalty = 0;
>
> if (irq == acpi_gbl_FADT.sci_interrupt)
> penalty += PIRQ_PENALTY_PCI_USING;
>
> penalty += acpi_irq_pci_sharing_penalty(irq);
> return penalty;
> }
>
> static int acpi_pci_link_allocate(struct acpi_pci_link *link)
> {
> unsigned int best = ~0;
> ...
>
> for (i = (link->irq.possible_count - 1); i >= 0; i--) {
> candidate = link->irq.possible[i];
> if (!pci_compatible_trigger(candidate))
> continue;
>
> penalty = acpi_irq_get_penalty(candidate);
> if (penalty < best) {
> irq = candidate;
> best = penalty;
> }
> }
> ...
> }
>
> This looks racy, because we test irq_get_trigger_type() without any
> kind of locking, and later acpi_register_gsi() calls
> irq_create_fwspec_mapping(), which looks like it sets the new trigger
> type. But I don't know how to fix that.
Right, if that pci link allocation code can be executed concurrent, then you
might end up with problem, but isn't that a problem even without
irq_get_trigger_type()?
Thanks,
tglx
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