[Discussion] how to implement external power down for ARM

[shannon.zhao@linaro.org (Shannon Zhao)]

On 2015/4/30 21:38, Hanjun Guo wrote:
> On 2015?04?30? 17:29, Mark Rutland wrote:
>> On Thu, Apr 30, 2015 at 10:08:05AM +0100, Peter Maydell wrote:
>>> On 30 April 2015 at 09:03, Arnd Bergmann <arnd@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?
>>
>> No, you're correct.
>>
>> There are a few "power button" bindings around, but it looks like they
>> all rely on platform details and aren't that generic.
>>
>> With ACPI what events may be singalled? Just power off, or reset, etc?
> 
> For the ACPI events, we generally have three events:
>  - sleep event
>  - power off event
>  - wakeup event
> 

In addition, we might need hotplug event later if we want to support
hotplug with DT.

>>
>> Which of these do we need to be able to handle with DT?
> 
> I think power off event is the most used event, sleep
> and wakeup events are often used for laptops.
> 
> For ACPI based power off, there will be a GPIO singled
> event (hardware reduced platform such as ARM64) or a
> special interrupt called SCI signaled to OS to let
> OS know there is a button event happened, then there is
> a driver handle this interrupt to tell the detail event,
> the driver will send the event to user space using input
> framework(deprecated) or netlink event.
> 
> I think DT can do it in a similar way, using input framework
> or netlink sending power button to user space and shutdown
> the system.
> 

Yes, we could do it like ACPI. But then they are almost same. I just
wonder the necessity of this.

> If any detail information needed, please let me know.
> 
> Thanks
> Hanjun

-- 
Shannon