[Discussion] how to implement external power down for ARM

[Peter Maydell]

On 30 April 2015 at 09:03, Arnd Bergmann <arnd at arndb.de> wrote:
> On Thursday 30 April 2015 09:03:22 Shannon Zhao wrote:
>>
>> I am looking at adding support for external power down and reboot to
>> ARM VMs.  With ACPI this is fairly straight forward and requires only
>> adding a GPIO controller to the virt machine model and extending ACPI
>> appropriately (see code here [1]). In addition Linaro LEG also have
>> done a test that uses GPIO as power button to shutdown OS on fast model
>> (see detail here [2]).
>>
>> However, we would like for this to work in systems that do not use
>> ACPI as well. Adding a GPIO controller will still work, but we would
>> need a generic way to tell Linux how to handle the GPIO events without
>> adding any board-specific code to the VIRT platform. And what guest
>> kernel driver do we need? Do we need another user-level daemon like acpid?
>>
>> Note that external shutdown can also be accomplished using the qemu
>> guest agent [3], but maybe this is not a sufficiently stable approach.
>>
>> Any input on the approach to take here is very welcome.
>
> I would expect drivers/power/reset/gpio-poweroff.c to work in an identical
> way with ACPI and DT, once you have added an ACPI binding for it.

Looking at Documentation/devicetree/bindings/gpio/gpio-poweroff.txt
that appears to be for "let the guest kernel turn off the system
from the inside by toggling a GPIO line". What we want is the
opposite: the external system (QEMU, in this case) toggles a GPIO
line in order to request the guest kernel to do a clean shutdown
or reboot. Or have I misunderstood what gpio-poweroff can do?

[I would expect the shutdown itself to be done via PSCI.]

thanks
-- PMM