Sensors, CPU & Fans Control

Sensors

CPU temperature

• To get CPU temperature, kernel modules are needed. I2C support in particular is generally required (compile I2C_CHARDEV).
• In addition, the correct modules corresponding to your chip should be added in the Hardware Monitoring support and I2C sections. One good technique is to compile everything available, then run sensors-detect, which will find the correct module. Then you can delete all the other modules and rebuild the kernel.
• For a recent Intel Core CPU, compile in the SENSORS_CORETEMP module (Device Drivers -> Hardware Monitoring support).
• In user space, you need to emerge lm-sensors.
• A KDE widget (plasmoid) is available to display current CPU temperature. You need to emerge ksysguard with the lm_sensors USE flag in order for it to work. It's not very beautiful but functional.
• For an Intel i7-11700, CPU temperatures of 40-60 degrees when doing standard operations (Firefox opened with 30 tabs, Dolphin, Konsole, a few other programs) seem standard. You should reach 35-45 degrees when completely idle (with a summer outside temperature). More is not normal and can indicate a CPU cooler issue or ventilation issue.

Fan metrics

• lm-sensors is also needed, along with some kernel modules (same procedure as for CPU temperature above).
• You may need to add the acpi_enforce_resources=lax command-line kernel boot parameter for some ASUS motherboards (like the ROG Strix Z590-I).
• You can then get the current fan RPM by running the sensors program.

Asus

• On some ASUS motherboards you may need to build the Asus atk0110 ACPI module. You don't need anything else but this module is not detected by sensors-detect; you must load it manually in the configuration file, see below.
• If you use the Asus atk0110 module, enter the following in /etc/conf.d/lm_sensors:

MODULE_0=asus_atk0110

Fan Control

• Fan control is better done in the BIOS. Normally it will allow you to set a fan profile, which associates a given CPU temperature with a given rotation speed. Usually you will want to keep rotation speed low to minimize noise, even if the CPU gets hot. In particular, running the fan at full speed produces a lot of noise at least on Noctua fans.
• CPU temperature should be less than 45 degrees while idling, 75 degrees under normal load, and 95 degrees under heavy load (gaming).

Pulse-Width Modulation

• Modern fans implement this interface, and in theory this makes it possible to control the fan speed from the OS rather than the BIOS. In practice, I think it's hard to do so and I was not able to even obtain metrics via PWM.
• Kernel modules that might be needed for this include Device Drivers -> Pulse-Width Modulation and Device Drivers -> Hardware Monitoring support -> PWM fan.

CPU Control

• Modern CPUs implement frequency scaling, which means they run at higher clocks under load and run slower when idle or under light load.
• In the Linux kernel, software support is implemented in the Power management and ACPI options -> CPU frequency scaling sections.

Intel processors

• Modern Intel CPUs (recent Core models) use the P-state driver. Documentation is available here.
• This driver only support two governors: performance and powersave. Building other governors into the kernel is useless, they won't be used.
• The performance governor produces high CPU temperatures that generally send the fans to high speeds, while powersave produces temperatures lower than 50 degrees.
• You can switch the current used governor by using the cpupower program:

cpupower frequency-set -g powersave

• You can obtain the currently running frequency and used governor by running:

cat /sys/devices/system/cpu/cpufreq/policy*/scaling_cur_freq
cat /sys/devices/system/cpu/cpufreq/policy*/scaling_governor

Updating CPU microcode

• It is recommended to update the CPU microcode at boot. With initramfs it can be done easily; add the initramfs USE flag and emerge sys-firmware/intel-microcode. It will automatically copy an intel-uc.img file to /boot. GRUB can then load this file via an initrd line normally.
• It's also possible to update the CPU microcode without initramfs support. In that case the correct firmware must be identified and built into the kernel. See this page for details.
• In any case, you can check if microcode was correctly loaded by looking at dmesg (should be the first log line).

Potential important kernel modules

• Some modules in the ACPI section (mostly for CPU frequency scaling).
• Some modules in Device Drivers -> Multifunction device drivers (in particular the Intel LPSS modules?).
• Some modules in Device Drivers -> X86 Platform specific drivers (Intel ones?).