[GTALUG] TIL: rtcwake / wakealarm

[D. Hugh Redelmeier]

| From: Stewart C. Russell via talk <talk at gtalug.org>

| https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=309093&p=1849326&sid=ca7a14f7d160f929378be4691b1aed9e#p1849291

Intereresting.  Thanks.

I always thought that shutting down a Pi really left it on.  That's one 
reason some people like Pi power supplies with a switch.  To what 
extent is that true?

How much power is saved?  I delayed sending this message, hoping to add 
information about this but I'm just not getting to the experiments.

Most PCs seem to have a firmware (BIOS) setting for when to wake up.  
I've never thought that useful since it required shutting the OS down and 
going into the firmware setup menu.

The rtcwake(8) command was explored by Giles on a PC.  It's available on 
my desktop.  The manpage refers to nvram-wakeup, but I don't see that on 
my system (fedora).  It seems to be part of debian.

There are a lot of things under /sys/class/rtc/rtc0 on my desktop.  I 
haven't searched for any documentation that might exist.

On a related topic:

Windows PCs seem to enter sleep mode when they haven't done anything for  
a certain amount of time.

My Linux boxes don't, as far as I've noticed.

Except for one Dell desktop box supplied with Ubuntu.  It does this too.

How does one configure conventional Linux to do this?

(I could probably spelunk systemd on that Dell system to see what's going 
on but I fear entering a Labyrinth and never coming out.)

Oh:

systemd-hibernate-resume (8) - Resume from hibernation
systemd-hibernate-resume-generator (8) - Unit generator for resume= kernel parameter
systemd-hibernate-resume at .service (8) - Resume from hibernation
systemd-hibernate.service (8) - System sleep state logic
systemd-suspend-then-hibernate.service (8) - System sleep state logic
systemd-sleep.conf (5) - Suspend and hibernation configuration file
systemd-suspend-then-hibernate.service (8) - System sleep state logic
systemd-suspend.service (8) - System sleep state logic

I haven't read these yet or looked for an over-arching document.