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Date:   Thu, 1 Aug 2019 10:13:31 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Aubrey Li <aubrey.intel@gmail.com>
cc:     Daniel Drake <drake@endlessm.com>, x86@kernel.org,
        "Li, Aubrey" <aubrey.li@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Endless Linux Upstreaming Team <linux@endlessm.com>
Subject: Re: setup_boot_APIC_clock() NULL dereference during early boot on
 reduced hardware platforms
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On Thu, 1 Aug 2019, Aubrey Li wrote:
> On Thu, Aug 1, 2019 at 3:35 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > On Thu, 1 Aug 2019, Aubrey Li wrote:
> > > On Thu, Aug 1, 2019 at 2:26 PM Daniel Drake <drake@endlessm.com> 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