Re: [PATCH V4 1/3] ACPI, PCI, IRQ: assign ISA IRQ directly during early boot stages

Bjorn Helgaas
2016-10-21T14:08:22Z

On Thu, Oct 20, 2016 at 08:39:30PM -0500, Bjorn Helgaas wrote:
> On Wed, Oct 19, 2016 at 06:21:02PM -0400, Sinan Kaya wrote:
> > The penalty determination of ISA IRQ goes through 4 paths.
> > 1. assign PCI_USING during power up via acpi_irq_penalty_init.
> > 2. update the penalty with acpi_penalize_isa_irq function based on the
> > active parameter.
> > 3. kernel command line penalty update via acpi_irq_penalty_update function.
> > 4. increment the penalty as USING right after the IRQ is assign to PCI.
> >
> > acpi_penalize_isa_irq and acpi_irq_penalty_update functions get called
> > before the ACPI subsystem is started.
> >
> > These API need to bypass the acpi_irq_get_penalty function.
>
> I don't mind this patch, but the changelog doesn't tell me what's
> broken and why we need this fix. Apparently acpi_irq_get_penalty()
> doesn't work before ACPI is initialized, but I don't see *why* it
> wouldn't work.
>
> However, I see one bug it *does* fix: we do not store the SCI penalty
> in the acpi_isa_irq_penalty[] table because acpi_isa_irq_penalty[]
> only holds ISA IRQ penalties, and there's no guarantee that the SCI is
> an ISA IRQ. But prior to this patch, we added in the SCI penalty to
> the acpi_isa_irq_penalty[] entry when the SCI was an ISA IRQ, which
> makes acpi_irq_get_penalty() return the wrong thing. Consider:
>
> Initially acpi_isa_irq_penalty[9] = 0.
> Assume sci_interrupt = 9.
> Then acpi_irq_get_penalty(9) returns X.
> If we call acpi_penalize_isa_irq(9, 1),
> it sets acpi_isa_irq_penalty[9] = X,
> and now acpi_irq_get_penalty(9) returns X + X.

Oops, I forgot the penalty we *intended* to add with
acpi_penalize_isa_irq(). It's really like this, where X is the SCI
penalty and Y is the part added by acpi_penalize_isa_irq():

Initially acpi_isa_irq_penalty[9] = 0.
Assume sci_interrupt = 9.
Then acpi_irq_get_penalty(9) returns X.
If we call acpi_penalize_isa_irq(9, 1),
it sets acpi_isa_irq_penalty[9] = X + Y,
and now acpi_irq_get_penalty(9) returns X + X + Y.

At the end, acpi_irq_get_penalty(9) *should* return X + Y, but instead
it returns X + X + Y, i.e., the SCI penalty is included twice.