Why C3 state isn't entered?

[Nate Lawson]

Andrey Smirnov wrote:
> Hello!
> 
> I've got Centrino notebook (specifically Asus M3700Np), and it doesn't 
> seem to enter C3 sleep state in any case.
> Sample output from 'sysctl hw.acpi.cpu' (while working at battery):
> 
> hw.acpi.cpu.cx_supported: C1/0 C2/1 C3/101
> hw.acpi.cpu.cx_lowest: C3
> hw.acpi.cpu.cx_usage: 0.00% 100.00% 0.00%
> 
> So C3 is supported, is allowed but isn't entered according to cx_usage. 
> Why could it be so?
> 
> My system:
> bash-2.05b$ uname -a
> FreeBSD smira.mlab.cs.msu.su 6.0-CURRENT FreeBSD 6.0-CURRENT #1: Wed Oct 
> 13 13:50:01 MSD 2004     
> root at smira.mlab.cs.msu.su:/usr/src/sys/i386/compile/smira.mobile  i386
> 
> acpidump -d -t output, bzipped:
> http://images.delit.net/acpidump.asl.bz2

This is a FAQ, perhaps we can add it to the handbook entry or man page:

C3 can't be entered while bus masters are active.  Typical bus mastering 
devices include USB and sound.  Since USB constantly polls when loaded, 
disabling USB support or stopping sound playback should allow C3 to be 
used.  A better fix would be to power down the USB host controller when 
idle.

-Nate