My Ubuntu server won't power off after shutdown. I have not managed to resolve the problem, but thought I'd share my notes. My system is a P4 2.0GHz, 256MB RAMBUS RAM. 

Description of the problem as follows. I try to shut down, and I get all the normal kernel messages. The system eventually halts, and the hard drives spin down, but the motherboard does not power off. If I try to force ACPI the kernel reports "ACPI: Unable to enable ACPI" at start-up.

Here are some of the things I've tried, unfortuately without success:

I've tried shutting down under Ubuntu Server 6.06.2, 7.04 and 7.10; all exhibit the same problem.

I've waited about 5-10 minutes after the "Will now halt" message appears, but the motherboard and fans are still powered up (although the HDDs have switched off). The last kernel message (under 7.10) is "halt: Unable to iterate IDE devices: No such file or directory"

I have tried shutdown -h now, shutdown -P now, poweroff, halt.

I have modified the kernel command line in /boot/grub/menu.lst. I've tried adding the following combinations:

	acpi=off noapic
	noacpi acpi=off apm=on noapic apm=power_off
	acpi=force lapic
	noapic reboot=b

I've also tried adding acpi=force: I got an error - ACPI: Unable to start ACPI. The following lines copied from /var/log/dmesg may contain clues...?

	[   16.118153] ACPI: Core revision 20070126
	[   16.118223] ACPI: Looking for DSDT in initramfs... error, file /DSDT.aml not found.
	[   16.119618] ACPI: setting ELCR to 0200 (from 0e28)
	[   16.605267] ACPI Error (hwacpi-0142): Hardware did not change modes [20070126]
	[   16.605275] ACPI Error (evxfevnt-0086): Could not transition to ACPI mode [20070126]
	[   16.605281] ACPI Warning (utxface-0139): AcpiEnable failed [20070126]
	[   16.605286] ACPI: Unable to enable ACPI

I have tried removing my network card (it's the only PCI card I'm using).

I've tried removing my graphics card, booting the machine headless.

I've tried booting Ubuntu in recovery mode (under 7.10) and shutdown doesn't work in there either.

I've checked and there doesn't seem to be anything abnormal about /etc/rc0.d/S90halt.

I tried to modprobe apm and it caused the system to hang. The last message output from the kernel is "Bad EIP value."

I tried manually stopping samba before trying to shutdown. I also tried unmounting samba shared drives prior to shutdown.

I installed the acpi package and ran it. This confirmed that the kernel had not been started with ACPI support. Installing and running acpid confirmed this as it couldn't find the acpi path (acpid: can't open /proc/acpi/event: No such file or directory).

I tried to modprobe apm and it caused the system to hang. The kernel generated quite a lot of output but I was unable to log this. The last message output from the kernel is "Bad EIP value."

I tried installing apmd: this crashed the system in the same way as starting the apm module...

Now here's the interesting thing: With the exact same system I've booted from an Ubuntu Live CD (7.10), and powering off from the desktop works! As does shutting down from the Terminal. Also, I've just installed Debian 4.0r2 server on exactly the same system, and running shutdown -h halts and powers off the system no problem. From this I have ruled out the possibility of hardware trouble, or the need to install a new BIOS.

This is as far as I've got. I've stopped short of recompiling the kernel, or installing a different one, although I read in a post somewhere that kernel 2.6.20-16 might work. I have a hunch that the problem may have something to do with interrupt requests, but I don't really know enough about it to investigate. And of course, that could be totally wrong ;)

Anyway, if I can provide any more useful information to help resolve this, please let me know. In the mean time, I hope that all of the above is in some way useful !

Cheers :)
James