[PATCH] arm64: acpi: add a Kconfig option to prefer ACPI boot over DT

[roy.lists@gmail.com (Roy Franz (HPE))]

On Tue, Apr 12, 2016 at 6:35 AM, Catalin Marinas
<catalin.marinas@arm.com> wrote:
> On Tue, Apr 12, 2016 at 03:19:58PM +0200, Ard Biesheuvel wrote:
>> On 12 April 2016 at 15:07, Catalin Marinas <catalin.marinas@arm.com> wrote:
>> > On Mon, Apr 11, 2016 at 01:19:28PM +0200, Ard Biesheuvel wrote:
>> >> If both ACPI and DT platform descriptions are available, and the
>> >> kernel was configured at build time to support both flavours, the
>> >> default policy in absence of a acpi=[off|force] kernel command line
>> >> parameter is to prefer DT over ACPI.
>> >>
>> >> This adds an option to invert that default policy, and prefer ACPI
>> >> over DT instead. Note that this policy is still superseded by the
>> >> value of the acpi= command line parameter.
>> >
>> > Why do we need another method to specify an ACPI boot? I thought those
>> > vendors going for ACPI wouldn't be bothered with DT anyway.
>> >
>> > I'm not keen on having kernel builds with different behavior in respect
>> > of whether ACPI or DT is preferred.
>>
>> How about adding support for acpi=on then? Currently, we only have
>> acpi=off or acpi=force, and there is no way (i.e., for a distro
>> installer) to boot via ACPI if it can but fall back to DT otherwise.
>> Some enterprise features (like RAS) depend on ACPI boot so it may
>> simply preferred but not mandated in some cases.
>
> Since this is a distro preference, I would rather have an acpi=on
> option.

While this is a 'distro preference', I think it is somewhat ugly for
this to be configured on
the commandline.   We (HPE) don't support DT, and I don't think that
is likely to change.
While we control the firmware for our main internal platform, and
don't provide a DT there,
we also do development and testing on other platforms where the
firmware may provide a DT,
but we never want to use it.  This requires developers/users to
specify "acpi=force" on the
command-line to boot in a supported manner.
I would much rather be able to configure the kernel to prefer (or even
unconditionally require)
ACPI to boot, as this will be the normal, default, and only supported
way to boot for our platform,
and I expect this to also be the case in much of the enterprise space.

Since I don't think it is possible to build an arm64 kernel with only
ACPI, and no DT support,
I think a kconfig option to select the preferred HW description to be
used is the better solution.

Roy


>
> --
> Catalin
>
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