Hi all,

I've been doing some power efficiency tuning on my Unraid 7.2.3 server and ran into what appears to be a kernel-level limitation. Wanted to document my findings and see if anyone else with Arrow Lake hardware has run into this and found a workaround.

System specs:

- CPU: Intel Core Ultra 5 225 (Arrow Lake, family 0x6, model 0xc6, stepping 0x2)

- Motherboard: ASUS Z890M Plus WiFi

- RAM: 24GB DDR5

- Unraid version: 7.2.3

- Kernel: 6.12.54-Unraid

The problem:

Despite having all C-states properly configured in BIOS (Package C State Limit = C10, Enhanced C-states = Enabled, CPU C-states = Enabled), powertop only shows C1/C2/C3 ACPI states. C6, C8, and C10 are never reached.

Diagnostics:

The cpuidle states exposed are all ACPI FFH MWAIT rather than native intel_idle states:

/sys/devices/system/cpu/cpu0/cpuidle/state0/desc: CPUIDLE CORE POLL IDLE

/sys/devices/system/cpu/cpu0/cpuidle/state1/desc: ACPI FFH MWAIT 0x0

/sys/devices/system/cpu/cpu0/cpuidle/state2/desc: ACPI FFH MWAIT 0x21

/sys/devices/system/cpu/cpu0/cpuidle/state3/desc: ACPI FFH MWAIT 0x60

The intel_idle driver is active (cat /sys/devices/system/cpu/cpuidle/current_driver returns intel_idle) but produces no initialization messages in dmesg beyond echoing the kernel command line. It is compiled into the kernel, not a loadable module:

find /lib/modules/$(uname -r) -name "intel_idle*" returns nothing.

dmesg confirms the CPU is identified correctly:

smpboot: CPU0: Intel(R) Core(TM) Ultra 5 225 (family: 0x6, model: 0xc6, stepping: 0x2)

However, intel_idle has no native C-state table entry for model 0xc6 in kernel 6.12, causing it to silently defer to the ACPI Cpu0Cst table, which only exposes C3 as the deepest state.

Steps already taken (none resolved the issue):

- BIOS: ASPM fully enabled (L1, L1.1, L1.2 on all PCIe sections)

- BIOS: VMD controller disabled and confirmed absent from dmesg

- Kernel parameter intel_idle.max_cstate=9 added to syslinux.cfg (confirmed in /proc/cmdline)

- powertop --auto-tune run at boot via user script

My best explanation so far:

Arrow Lake desktop (model 0xc6) support was added to the intel_idle driver in kernel 6.13. Unraid's current kernel 6.12.54 predates this, so intel_idle falls back to ACPI C-state definitions which cap at C3.

Has anyone else with Arrow Lake or Core Ultra 200 series hardware run into this? Were you able to work around it, or are you also stuck waiting on a kernel update? Any input appreciated!

1. Intel is lazy so they will never fix intel_idle. Note: acpi_idle gives lower power consumption

2. powertop will never work; see #1. Read above thread, and it explains c-states are working; reporting is just busted.

3. zero OEM's will enable S0 on their Arrow Lake motherboards. Try reaching anyone at Asus and ask for S0 be an option in BIOS. 7-10w drop just from that.

4. Every single intel microcode improvement drives up power consumption; roll back your BIOS and watch your power drop.