[Discussion] how to implement external power down for ARM

[Christoffer Dall]

On Tue, May 5, 2015 at 11:53 AM, Arnd Bergmann <arnd at arndb.de> wrote:
> On Tuesday 05 May 2015 12:27:54 Joel Stanley wrote:
>> On Tue, May 5, 2015 at 1:19 AM, Arnd Bergmann <arnd at arndb.de> wrote:
>> > On Monday 04 May 2015 10:09:04 Shannon Zhao wrote:
>> >> On 2015/4/30 17:56, Arnd Bergmann wrote:
>> >> > On Thursday 30 April 2015 10:29:53 Mark Rutland wrote:
>> >
>> >> > a) have an input device send the KEY_POWEROFF or KEY_REBOOT events to
>> >> >    user space, and have some process (e.g. desktop environment, or daemon)
>> >>
>> >> About this daemon, do we need a doc or spec to standardize it?
>> >
>> > There is probably documentation about this that we just need to locate.
>> > On my workstation, the power button device is owned by "systemd-logind",
>> > I'd assume something like that to be present elsewhere as well.
>> >
>> >> > b) Have a special driver that calls orderly_poweroff or orderly_reboot.
>> >> >    Only PowerPC (OPAL) uses the orderly_reboot() here, but a few platforms
>> >> >    (Xen, fsl_hypervisor, and some sparc and powerpc machines) as well
>> >> >    as some device drivers (thermal management etc) call this as well.
>> >> >    The effect is to call a user-configured binary (/sbin/reboot and
>> >> >    /sbin/poweroff by default).
>> >> >
>> >>
>> >> This looks good which reuses existing user space process.
>> >>
>> >
>> > Joel Stanly wrote that patch for PowerPC, maybe he can explain why
>> > this PowerPC is doing it this way instead of having an input device
>> > that sends KEY_RESTART. It's probably a good idea to have ARM and
>> > PowerPC normally do the same thing here, especially if there are
>> > problems with just using a gpio-key device.
>>
>> We wanted to support reboot and poweroff on existing distributions
>> without having to rely on new userspace.
>>
>> I chatted to a few people at linux.conf.au this year and we came to
>> the conclusion that the options were orderly_poweroff() or ACPI for
>> doing a graceful shutdown involving userspace. ACPI required acpid or
>> a desktop environment listening to the netlink message (plus we aren't
>> an ACPI machine) so that was out.
>>
>> KEY_POWEROFF looks useful, but how would you support userspace that
>> does not have a systemd-logind, or anything listening to that input
>> event?
>
> Ok, thanks for providing that background. I was under the impression
> that the power button was not handled through netlink but instead
> through the input subsystem, and I just checked the acpid source
> code and found that it basically handles acpi events and input
> events separately and should still work fine if only one of the
> two is present.
>
> I still find it hard to come up with a good decision for ARM here:
> I think either way will work in most cases but has minor downsides
> (orderly_poweroff() and orderly_reboot() being less configurable
> vs input events requiring some user space to run).
> Unfortunately, it is not an option to just "do both", because the
> orderly_reboot() will unconditionally reboot the system, so any
> user configuration is just ignored here.
>
> Maybe we just use the the input framework for now, because it
> requires zero kernel changes and is already used on ARM platforms
> today, and see if we run into problems with that.
>
Do ARM distributions (Fedora, Ubuntu, Debian, ...) support the
necessary userspace handling so that things work with the input
subsystem?

-Christoffer