Re: setup_boot_APIC_clock() NULL dereference during early boot on reduced hardware platforms

[Thomas Gleixner]

On Thu, 1 Aug 2019, Aubrey Li wrote:
> On Thu, Aug 1, 2019 at 3:35 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, 1 Aug 2019, Aubrey Li wrote:
> > > On Thu, Aug 1, 2019 at 2:26 PM Daniel Drake <drake@xxxxxxxxxxxx> wrote:
> > > > global_clock_event is NULL here. This is a "reduced hardware" ACPI
> > > > platform so acpi_generic_reduced_hw_init() has set timer_init to NULL,
> > > > avoiding the usual codepaths that would set up global_clock_event.
> > > >
> > > IIRC, acpi_generic_reduced_hw_init() avoids initializing PIT, the status of
> > > this legacy device is unknown in ACPI hw-reduced mode.
> > >
> > > > I tried the obvious:
> > > >  if (!global_clock_event)
> > > >     return -1;
> > > >
> > > No, the platform needs a global clock event, can you turn on some other
> >
> > Wrong. The kernel boots perfectly fine without a global clock event. But
> > for that the TSC and LAPIC frequency must be known.
> 
> I think LAPIC fast calibrate is only supported on intel platform, while
> Daniel's box is an AMD platform. That's why lapic_init_clockevent() failed
> and fall into the code path which needs a global clock event.

We know that.

The point is that it does not matter which vendor a CPU comes from. The
kernel does support legacyless boot when the frequencies are known. Whether
that's currently possible on that particular CPU is a different question.

Thanks,

	tglx