There wasis a great summary on this matter here, but is not available anymore: http://www.macfreek.nl/memory/Reboot_Mac_running_Linux_after_power_failurehttps://web.archive.org/web/20230218203824/http://www.macfreek.nl/memory/Reboot_Mac_running_Linux_after_power_failure
In a nutshell, the configuration of the Advanced Configuration and Power Interface (ACPI) depends on the model of southbridge used in the Mac.
In order to configure auto-boot on power failure identify your southbridge model by looking up your LPC devices and create a cron tab or systemd service to set the bit registers accordingly.
$ lspci | grep LPC
For a Mac Mini early 2006 / MacMini1,1 / A1176:
00:1f.0 ISA bridge: Intel Corporation 82801GBM (ICH7-M) LPC Interface Bridge (rev 02)
For a Mac Mini early 2009 / MacMini3,1 / A1283:
00:03.0 ISA bridge: NVIDIA Corporation MCP79 LPC Bridge (rev b2)
For a Mac Mini early 2010 / MacMini4,1 / A1347:
00:03.0 ISA bridge: NVIDIA Corporation MCP89 LPC Bridge (rev a2)
For a Mac Mini Server 2011:
00:1f.0 ISA bridge: Intel Corporation HM65 Express Chipset Family LPC Controller (rev 05)
Please note that the configurations are not persistent across reboots so it requires configuration on startup (for example through systems as suggested by @Juan Carlos Méndez).
To create a systemd startup service, create a file /etc/systemd/system/reboot_on_power_failure.service:
[Unit]
Description=Reboot after power failure
[Service]
Type=oneshot
# Please select the appropriate ISA bridge for your MAC below:
# reboot register for Mac Mini with nVidia ISA bridge
# ExecStart=setpci -s 00:03.0 0x7b.b=0x19
# reboot register for Mac Mini with Intel ISA bridge
# ExecStart=sudo setpci -s 0:1f.0 0xa4.b=0
# reboot register for PPC Mac Mini (not tested myself):
# ExecStart=echo server_mode=1 > /proc/pmu/options
[Install]
WantedBy=sysinit.target
And run sudo systemctl enable --now reboot_on_power_failure.service to enable it.
