Re: [PATCH] x86/i8259: Work around buggy legacy PIC

Thomas Gleixner
2021-05-17T23:27:07Z

On Mon, May 17 2021 at 21:25, Maximilian Luz wrote:
> On 5/17/21 8:40 PM, Thomas Gleixner wrote:
>> Can you please add "apic=verbose" to the kernel command line and provide
>> full dmesg output for a kernel w/o your patch and one with your patch
>> applied?
>
> I don't actually own an affected device, but I'm sure Sachi can provide
> you with that.

Ok.

> As far as we can tell, due to the NULL PIC being chosen nr_legacy_irqs()
> returns 0. That in turn causes mp_check_pin_attr() to return false
> because is_level and active_low don't seem to match the expected
> values.

Ok.

> That check is essentially ignored if nr_legacy_irqs() returns a high
> enough value.

Close enough.

> I guess that might also be a firmware bug here? Not sure where the
> expected values come from.

They come from the interrupt override ACPI table and if not supplied
then irq 0-15 is preset with default values, which are type=edge and
polarity=high, i.e. the opposite of what the failing driver wants.

The ACPI table lacks an override entry for IRQ7. I looked at one of the
dmesg files in that github thread and that has overrides:

[ 0.111674] ACPI: INT_SRC_OVR (bus 0 bus_irq 0 global_irq 2 dfl dfl)
[ 0.111681] ACPI: INT_SRC_OVR (bus 0 bus_irq 9 global_irq 9 low level)
[ 0.111688] ACPI: IRQ0 used by override.
[ 0.111692] ACPI: IRQ9 used by override.

IRQ7 should have a corresponding entry as IRQ9 has:

https://github.com/linux-surface/acpidumps/blob/4da0148744164cea0c924dab92f45842fde03177/surface_laptop_4_amd/apic.dsl#L178

Subtable Type : 02 [Interrupt Source Override]
Length : 0A
Bus : 00
Source : 07
Interrupt : 00000007
Flags (decoded below) : 000F
Polarity : 3
Trigger Mode : 3

> Sachi can probably walk you through this a bit better as she's the one
> who tracked this down. See also [1, 2] and following comments.

Impressive detective work!

Sachi, can you please try the hack below to confirm the above?

It's not meant to be a solution, but it's the most trivial way to
validate this.

I'm pretty sure that Windows on Surface does not care about the PIC at
all. Whether that's on purpose to safe power or just because Windows
ignores the PIC completely by now does not matter at all. No idea how
that repeated poking on the PIC makes it come alive either and TBH, I
don't care too much about it simply because Linux is able to cope with a
missing PIC as long as the ACPI tables are correct.

I'm way too tired to think about a proper solution for that problem and
I noticed another related issue in that dmesg output:

[ 0.272448] Failed to register legacy timer interrupt

It's not a problem which causes failures, but it's related to the
missing PIC.

Needs some more thoughts with brain awake...

Thanks,

tglx
---
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -21,6 +21,7 @@
#include <linux/efi-bgrt.h>
#include <linux/serial_core.h>
#include <linux/pgtable.h>
+#include <linux/dmi.h>

#include <asm/e820/api.h>
#include <asm/irqdomain.h>
@@ -1155,6 +1156,17 @@ static void __init mp_config_acpi_legacy
}
}

+static const struct dmi_system_id surface_quirk[] __initconst = {
+ {
+ .ident = "Microsoft Surface Laptop 4 (AMD)",
+ .matches = {
+ DMI_MATCH(DMI_SYS_VENDOR, "Microsoft Corporation"),
+ DMI_MATCH(DMI_PRODUCT_SKU, "Surface_Laptop_4_1952:1953")
+ },
+ },
+ {}
+};
+
/*
* Parse IOAPIC related entries in MADT
* returns 0 on success, < 0 on error
@@ -1212,6 +1224,11 @@ static int __init acpi_parse_madt_ioapic
acpi_sci_ioapic_setup(acpi_gbl_FADT.sci_interrupt, 0, 0,
acpi_gbl_FADT.sci_interrupt);

+ if (dmi_check_system(surface_quirk)) {
+ pr_warn("Surface hack: Override irq 7\n");
+ mp_override_legacy_irq(7, 3, 3, 7);
+ }
+
/* Fill in identity legacy mappings where no override */
mp_config_acpi_legacy_irqs();