[systemd-devel] Intercepting/Delaying the boot process

[Lennart Poettering]

On Fr, 08.04.22 09:01, Andreas Hartmann (hartan at 7x.de) wrote:

> Hi,
>
>
> I've been owning a Pinephone for a while now and one thing that I find rather
> annoying is that whenever I plug the charger in, the thing will perform a full
> boot. I often find myself just wanting to plug it in without performing a full
> boot, i.e. only to have it charge and maybe see how far it has charged already.
>
> To this end I was wondering whether it would be feasible to "hook" into the boot
> process, somewhere before the disks are decrypted, to only have it charge (or
> continue to boot when I press some magic button maybe).

The way I'd implement this would be via a generator. See
https://www.freedesktop.org/software/systemd/man/systemd.generator.html
for details on that.

Generators are in a way "plugins" to systemd's unit graph logic. They
can insert units and dependencies into the tree from external
sources. They can also override the regular tree. So here's what I
would specifically do: write a tiny generator that figures out if it
is in this charging mode. And if so, it overrides the "default.target"
symlink so that it points to a new target "charge-mode.target" or
so. Note that "default.target" is the default target the system will
boot into. Thus with this generator you made sure that if this charge
mode is selected only a tiny subset of units can be activated. Now,
add in a service into "charge-mode.target" that brings some nice
graphics to screen, and then just waits. If eventually something
happens so that the system shall power up fully, you can do the
equivalent of "systemctl isolate graphical.target" so that you boot up
into the regular system again.

> - After which stage in the boot process is the sysfs available?

sysfs is mounted by very early PID 1 code. It's basically always there.

> - Can I delay the boot for an indefinite amount of time, or will this cause some
> services later on in the process to timeout and fail?

You can, but as mentioned I'd rather redirect boot to a separate
target, see above.

> - Is it possible for a service during early boot to command a system shutdown
> instead of continuing to boot?

Sure.

> - May I simply take control of the TTY and clear/rewrite it as I like, or does
> systemd use some magic for this?

Use StandardInput=tty and your program will be started once there is
noone else who is the controller of the TTY.

This reminds of an RFE we have had for a while, and which I think
would make sense to add directly to systemd: a generator that detects
whether the battery is nearly empty at boot, and if so redirects boot
to some special target showing a nice message for a bit and then
powering off.

How do you intend to discern "power-up due to charger plugged in" from
"power-up due to power button pressed" btw? Is there generic infra for
discerning that? Or just something pinephone specific thing? Ideally
we had some generic infra for that in the kernel.

(On PCs there's a field for that in DMI called "Wake-up Type". I
wonder how well that works these days.)

Lennart

--
Lennart Poettering, Berlin