[systemd-devel] System power control

[David Lambert]

On 08/12/2012 06:09 PM, Lennart Poettering wrote:
> On Sun, 12.08.12 17:55, David Lambert (dave at lambsys.com) wrote:
>
>> I am developing a battery backed up cape for a Beaglebone project
>> which uses systemd under Angstrom. In order to conserve battery
>> life, I would like to signal my battery control hardware via a GPIO
>> when system shutdown is complete and all file systems have been
>> dismounted. Is there a "hook" in systemd which will allow me to call
>> a small user helper program at this point. Looking at the shutdown.c
>> source, I see the following code:
>>
>>          if (access("/run/initramfs/shutdown", X_OK) == 0) {
>>
>>                  if (prepare_new_root() >= 0 &&
>>                      pivot_to_new_root() >= 0) {
>>                          execv("/shutdown", argv);
>>                          log_error("Failed to execute shutdown binary: %m");
>>                  }
>>          }
>>
>> Is the intention of this code to provide such a hook? Can I cheat by
>> creating /run/initramfs/ just prior to shutdown and copy a small
>> shutdown executable of my own here. This sounds like it may work,
>> but seems like a terrible hack. I feel that there should be a more
>> elegant way to achieve this goal.
> No, that's for the initrd to undo the storage setup it did at boot.
>
> If you look a tiny bit above the code you quoted you will find the right
> way to do this: right before shutdown we execute all executables from
> /usr/lib/systemd/system-shutdown/. Just drop int your stuff thre, mark
> it executable, and you are done.
>
> This is documented in systemd-halt.service(8) in more detail.
>
> Note that using this for powering down things is a racy and unsafe
> though. Some things maintained by the kernel are only synced to disk
> when the actual kernel system call reboot(2) is invoked. That means that
> you should be careful only to shut off power after that system call has
> been entered and finished this syncing bit. Effectively this means that
> you need to do your power down logic in the kernel, as this cannot be
> done safely and race-freely from usersapce.
>
> Hope this is helpful.
>
> Lennart
>
Thanks again Lennart. One thing that got me confused is that on my 
system (Angstrom) the /usr/lib/systemd/... has been replaced by 
/lib/systemd/... :-(