AMD engineers have begun ironing out ACPI C4 support for the Linux kernel in order to provide greater power savings on newer AMD Ryzen platforms that have begun to support this deeper sleep state.
Sent out to the Linux kernel mailing list yesterday were the initial patches for preparing ACPI C4 support within the ACPI “processor_idle” driver.
AMD Linux engineer Mario Limonciello commented on the patch series:
“Some AMD systems have introduced support for ACPI C4. Systems that support ACPI C4 will enter CPU OFF at ACPI C3 and VDD OFF at ACPI C4.
VDD OFF is a requirement for s0i3, and thus if C4 isn’t supported then a system can’t enter s0i3.
Add support for ACPI C4 to the processor idle driver, following similar semantics of ACPI C3.”
He further added on the ACPI processor idle driver patch:
“Currently the Linux kernel supports up to ACPI C3, and if a system supports any further states they are ignored. This causes problems on some AMD hardware which can support up to ACPI C4.
AMD systems that support up to C3 will enter CPUOFF and VDDOFF when entering C3. Systems that support up to C4 will enter CPUOFF when going to C3 and will enter VDDOFF when entering into C4.”
The patches are now under review in working their way toward the mainline Linux kernel for further enhancing power management on modern AMD hardware.
