[coreboot] What is needed in dsdt to enable S4(Besides \_S4)

[Bao, Zheng]

I have _S4 in my dsdt.asl.  But the dmesg only reports:

ACPI: (supports S0 S1 S2 S3 S5)

And cat /sys/power/state only says:

Standby mem

So what else is needed?

Thanks.

Zheng