[PATCH 19/19] Documentation: ACPI for ARM64

[Hanjun Guo]

On 2014-8-14 7:41, Rafael J. Wysocki wrote:
> On Tuesday, August 12, 2014 07:23:47 PM Catalin Marinas wrote:
>> On Mon, Jul 28, 2014 at 07:27:52PM +0100, Olof Johansson wrote:
>>> On Mon, Jul 28, 2014 at 10:00 AM, Mark Rutland <mark.rutland at arm.com> wrote:
>>>> On Mon, Jul 28, 2014 at 05:27:50PM +0100, Olof Johansson wrote:
>>>>> On Mon, Jul 28, 2014 at 11:07:50AM +0200, Arnd Bergmann wrote:
>>>>>> On Saturday 26 July 2014 19:34:48 Olof Johansson wrote:
>>>>>>> On Thu, Jul 24, 2014 at 6:00 AM, Hanjun Guo <hanjun.guo at linaro.org> wrote:
>>>>>>>> +Relationship with Device Tree
>>>>>>>> +-----------------------------
>>>>>>>> +
>>>>>>>> +ACPI support in drivers and subsystems for ARMv8 should never be mutually
>>>>>>>> +exclusive with DT support at compile time.
>>>>>>>> +
>>>>>>>> +At boot time the kernel will only use one description method depending on
>>>>>>>> +parameters passed from the bootloader.
>>>>>>>
>>>>>>> Possibly overriden by kernel bootargs. And as debated for quite a
>>>>>>> while earlier this year, acpi should still default to off -- if a DT
>>>>>>> and ACPI are both passed in, DT should at this time be given priority.
>>>>>>
>>>>>> I think this would be harder to do with the way that ACPI is passed in
>>>>>> to the kernel. IIRC, you always have a minimal DT information based on
>>>>>> the ARM64 boot protocol, but in the case of ACPI, this contains pointers
>>>>>> to the ACPI tables, which are then used for populating the Linux platform
>>>>>> devices (unless acpi=disabled is set), while the other contents of the
>>>>>> DTB may be present but we skip the of_platform_populate state.
>>>>>
>>>>> How can it be harder to do? If you support acpi=off, then you should support
>>>>> acpi=on.
>>>>>
>>>>> Another alternative would be to have an early fixup that stubs out
>>>>> the acpi properties from the DTB unless there's an 'acpi' or 'acpi=on'
>>>>> argument on the cmdline. Not quite as tidy a solution, though.
>>>>
>>>> I don't follow:
>>>>
>>>> If you want to disable ACPI, you can pass acpi=off. This is your
>>>> workaround for broken ACPI (and only if you happen to have wrirten your
>>>> own DTB, because many/most ACPI systems WILL NOT have a DTB to begin
>>>> with).
>>>
>>> All ACPI should be assumed broken at this time, so acpi=off _must_ be
>>> the default.
>>
>> (catching up with emails after holiday and I may have missed some of
>> your arguments)
>>
>> If we consider ACPI unusable on ARM but we still want to start merging
>> patches, we should rather make the config option depend on BROKEN
>> (though if it is that unusable that no real platform can use it, I would
>> rather not merge it at all at this stage).
> 
> I agree here.
> 
> I would recommend creating a separate branch for that living outside of the
> mainline kernel and merging it when there are real users.

Real users will coming soon, we already tested this patch set on real hardware
(ARM64 Juno platform), and I think ARM64 server chips and platforms will show
up before 3.18 is released.

For this patch set, DT is the first class citizen at now:

a) We can always set CONFIG_ACPI as off in Kconfig, and use DT only;

b) Even if we set CONFIG_ACPI=Y, we also can use DT as normal:

  - Pass DT blob without (valid) ACPI tables (just as we boot the kernel now),
    ACPI will disabled in the very early stage and FDT will still to be
    unflattened, so will not break DT booting.

  - We can pass ACPI=off to disable ACPI and use DT even if we got valid
    ACPI tables (in the v1 patch set);

So it is safe for people who want to use DT, and didn't change any behavior
of DT booting except some extra test of if(acpi_disabled).

Thanks
Hanjun