On Fri, Apr 8, 2022 at 10:08 AM Andreas Hartmann <hartan@xxxxx> 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). Looking at the order of
service startup I was thinking about maybe intercepting the boot between "local-
fs-pre.target" and "machines.target" because nothing happens there on my setup.
That "between" doesn't make sense, because the boot process waits for machines.target *in parallel* with waiting for all other services that are started via multi-user. The systemd boot process is non-linear and doesn't have a fixed order, it's a dependency graph.
According to bootup(7), there are two main synchronization points, sysinit.target and basic.target, which all non-"early" services will wait for. So if you have a service with Before=basic.target, it'll delay startup of all normal services. (It would then need to explicitly specify After= for things it needs, like specific devices or sockets.)
If the device was using an initramfs (which has a fully separate boot process, its own udevd, etc.) then you could make the initramfs decide between starting a normal boot vs minimal boot depending on charger status. If it doesn't have an initramfs, then systemd *generators* could also be used to dynamically swap default.target (or alter units in general) depending on some condition, as they also run after /sys is mounted but before any units at all are started... though this would only work if your wanted sysfs entries appear without needing udev.
For this to work ideally I would like to have the sysfs available so I don't have
to interact with the kernel directly. So here are my questions:
- After which stage in the boot process is the sysfs available?
It is mounted by init before any units are started. (sysfs *is* how you interact with the kernel though...)
But that doesn't mean all *devices* are available in it – many things could take some time to appear, and devices requiring drivers to be loaded as modules would only become available some unspecified time after systemd-udevd.service is started.
If you need a specific /sys or /dev device that's not guaranteed to be available statically, the correct way would be to use an After= dependency on that specific device (like After=dev-sda.device). Systemd relies on udev events for this.
- 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?
In theory you could, similar to how systemd-fsck works, but I'm not sure if that's the right way to do what you want.
- Is it possible for a service during early boot to command a system shutdown
instead of continuing to boot?
- May I simply take control of the TTY and clear/rewrite it as I like, or does
systemd use some magic for this?
Thank you in advance!
hartan
Mantas Mikulėnas
