[systemd-devel] Implementing resume from hibernation as a systemd unit file

[Ivan Shapovalov]

On Thursday 14 August 2014 at 03:10:30, Lennart Poettering wrote:	
> On Mon, 28.07.14 14:06, Ivan Shapovalov (intelfx100 at gmail.com) wrote:
> 
> > Hello all,
> > 
> > I'm trying to implement resuming from hibernation with systemd-based initramfs,
> > i. e. in terms of systemd unit files. That is, there should be a unit which
> > waits for the resume device and writes its major:minor into /sys/power/resume.
> > 
> > I've chosen to write a templated resume@<path>.service and a generator which
> > parses 'resume=' kernel parameter and enables a corresponding instance of the
> > resume at .service unit.
> > 
> > The resume at .service itself shall be ordered so that it is activated when there
> > are no filesystems mounted read-write, except tmpfs instances. It will be good
> > if it works both from initramfs and from real rootfs (if it is mounted RO).
> > 
> > So, here are some questions:
> > - is it the way to go (overall)?
> > - is it OK to use generators for outlined purpose?
> > - how to order resume at .service and where to symlink it?
> > 
> > I'm using Arch, if that matters -- but, IIUC, Arch strives to do everything
> > "in the upstream's way", so there should be no distro-specific details.
> 
> I am not entirely sure how the hibernation logic precisely works, but
> wouldn't it be possible to write a little udev rule (maybe with a
> helper) that adds a special device node symlink to the resume partition,
> so that /dev/disk/resume or so always points to the resume partition to
> use?
> 
> That udev rule should then maybe also pull in a unit that acts on
> it. But that unit doesn't have to be an instance, could just be a normal
> one...
> 
> Does that make any sense?
> 
> Lennart

The udev rule should be possible (provided that udevd does not need rootfs
remounted read-write -- I'd like to preserve some decency towards initrd-less
systems), but udev is a framework for handling events, whereas we don't have
any events here: such symlink can be derived from kernel command-line alone,
statically.

Yes, a udev rule would allow to create a symlink which is tracked by systemd,
so the dev-disk-resume.device appears and then it can be easily After='ed
from the resume unit, but... really, is udev the proper tool for this?

Actually, the main question is how to order the resume unit. It needs to run
before any real filesystems are mounted (speaking in terms of initrd) AND before
rootfs is remounted read-write (speaking in terms of initrd-less system), but
after whatever is needed to make the device node appear.

Thanks,
-- 
Ivan Shapovalov / intelfx /
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