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

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Tue, May 18, 2021 at 01:27:02AM +0200, Thomas Gleixner wrote:
> 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.

But ACPI has a pretty nice means about missing legacy hardware, it's called
Hardware Reduced mode. It excludes automatically the (legacy) PIC, PIT, etc.

OTOH it excludes ACPI power chip as well. I haven't looked into this, just
share my thoughts what else can be checked. (On Intel the MID devices use
that approach)

> Needs some more thoughts with brain awake...

-- 
With Best Regards,
Andy Shevchenko