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Subject: Re: setup_boot_APIC_clock() NULL dereference during early boot on
 reduced hardware platforms
To:     Thomas Gleixner <tglx@linutronix.de>,
        Aubrey Li <aubrey.intel@gmail.com>
Cc:     Daniel Drake <drake@endlessm.com>, x86@kernel.org,
        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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 <alpine.DEB.2.21.1908011011250.1788@nanos.tec.linutronix.de>
From:   "Li, Aubrey" <aubrey.li@linux.intel.com>
Message-ID: <81666b28-d029-56c3-8978-90abc219d1b7@linux.intel.com>
Date:   Thu, 1 Aug 2019 16:21:31 +0800
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On 2019/8/1 16:13, Thomas Gleixner wrote:
> 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.
> 
Yeah, I should specify, Daniel, your platform needs a global clock event, ;-)

Thanks,
-Aubrey