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From:   Daniel Drake <drake@endlessm.com>
Date:   Thu, 1 Aug 2019 17:00:23 +0800
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Subject: Re: setup_boot_APIC_clock() NULL dereference during early boot on
 reduced hardware platforms
To:     Aubrey Li <aubrey.intel@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>, 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>
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On Thu, Aug 1, 2019 at 3:16 PM Aubrey Li <aubrey.intel@gmail.com> wrote:
> No, the platform needs a global clock event, can you turn on some other
> clock source on your platform, like HPET?

Thanks Audrey and Thomas for the quick hints!

I double checked under Windows - it seems to be using a HPET there.
Also there is the HPET ACPI table. So I think this is the right angle
to look at.

Under Linux, hpet_legacy_clockevent_register() is the function where
global_clock_event can be set to HPET.

However, the only way this can be called is from hpet_enable().

hpet_enable() is called from 2 places:
 1. From hpet_time_init(). This is the default x86 timer_init that
acpi_generic_reduced_hw_init() took out of action here.
 2. From hpet_late_init(). However that function is only called late,
after calibrate_APIC_clock() has already crashed the kernel. Also,
even if moved earlier it would also not call hpet_enable() here
because the ACPI HPET table parsing has already populated
hpet_address.

I tried slotting in a call to hpet_enable() at an earlier point
regardless, but I still end up with the kernel hanging later during
boot, probably because irq0 fails to be setup and this error is hit:
    if (setup_irq(0, &irq0))
        pr_info("Failed to register legacy timer interrupt\n");

I'll go deeper into that; further hints welcome too.

Thanks
Daniel