Re: [SeaBIOS] [PATCH] acpi: reintroduce LNKS

[Laszlo Ersek]

On 12/13/12 08:42, Paolo Bonzini wrote:
> Commit 4540409 (acpi: LNKS is not needed, 2012-08-07) removed LNKS because
> it basically worked by chance: _CRS returns something else then one of the

^
("than")
> possible resources from _PRS, _DIS would not really disable the interrupt,
> and there was no _SRS method.  It just happened to work because all OSes
> have some kind of special-casing for SCI.
>
> Unfortunately, the code after the patch is also against the spec, and it
> breaks FreeBSD because it treats IRQ 9 polarity as active low without
> the Interrupt() entry.  Actually, numeric _PRT entries are handled the
> same in Linux and FreeBSD (as active-low).  However, under Linux it just
> happens to trigger another special casing of SCI which sets SCI up from
> its override entry in the MADT, ignoring the DSDT completely.

I won't pretend I understand what I'm talking about, but the ACPI spec
5.0 says in "5.2.12.5 Interrupt Source Override Structure",

Interrupt Source Overrides are also necessary when an identity
    mapped interrupt input has a non-standard polarity.

Hence "necessary but not sufficient", is that it?

OTOH, one might argue that the spec requires this override (if
appropriate) because it expects OSPM to look at it, and to base
decisions on it. There's also a

Note: You must have an interrupt source override entry for the IRQ
    mapped to the SCI interrupt if this IRQ is not identity mapped.
    This entry will override the value in SCI_INT in FADT. [...]

SCI_INT in the FADT is explained as

[...] OSPM is required to treat the ACPI SCI interrupt as a
    sharable, level, active low interrupt.

which is then overridden in the MADT, stating active-high polarity.

... I think the (current) Linux functions you mean are in
"arch/x86/kernel/acpi":

acpi_parse_int_src_ovr()
  acpi_sci_ioapic_setup()

The bit that fetches the polarity from the override dates back to 2004:

- https://bugzilla.kernel.org/show_bug.cgi?id=1622
-
http://git.kernel.org/?p=linux/kernel/git/tglx/history.git;a=commitdiff;h=e1...

How does FreeBSD handle the override?
> This patch adds back the LNKS, but without using the PIIX register for
> LNKA in its methods.
>
> Tested-by: Luigi Rizzo 
> Signed-off-by: Paolo Bonzini
>
> src/acpi-dsdt.dsl | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/src/acpi-dsdt.dsl b/src/acpi-dsdt.dsl
> index 8019b71..b58ef62 100644
> --- a/src/acpi-dsdt.dsl
> +++ b/src/acpi-dsdt.dsl
> @@ -187,7 +187,7 @@ DefinitionBlock (
>              prt_slot0(0x0000),
>              /* Device 1 is power mgmt device, and can only use irq 9 */
>            Package() { 0x1ffff, 0,    0, 9 },
>            Package() { 0x1ffff, 0, LNKS, 0 },
>            Package() { 0x1ffff, 1, LNKB, 0 },
>            Package() { 0x1ffff, 2, LNKC, 0 },
>            Package() { 0x1ffff, 3, LNKD, 0 },

Would it be possible to restore the prt_slot() macro here that was
removed by commit 4540409? The new code seems to match what the macro
would expand to.
> @@ -278,6 +278,22 @@ DefinitionBlock (
>          define_link(LNKB, 1, PRQ1)
>          define_link(LNKC, 2, PRQ2)
>          define_link(LNKD, 3, PRQ3)
>    Device(LNKS) {
>        Name(_HID, EISAID("PNP0C0F"))
>        Name(_UID, 4)
>        Name(_PRS, ResourceTemplate() {
>            Interrupt(, Level, ActiveHigh, Shared) { 9 }
>        })
>        // The SCI cannot be disabled and is always attached to GSI 9,
>        // so these are no-ops.  We only need this link to override the
>        // polarity to active high and match the content of the MADT.
>        Method(_STA, 0, NotSerialized) { Return (0x0b) }
>        Method(_DIS, 0, NotSerialized) { }
>        Method(_CRS, 0, NotSerialized) { Return (_PRS) }
>        Method(_SRS, 1, NotSerialized) { }
>    }
> }
>
> #include "acpi-dsdt-cpu-hotplug.dsl"

Reviewed-by: Laszlo Ersek

although I'm not sure FreeBSD does the right thing.