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

["Li, Aubrey" <aubrey.li@linux.intel.com>]

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