Commit bd2d4202 authored by Bastien Nocera's avatar Bastien Nocera
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README: Document how the Intel P-State driver works

It used to just tweak a single file, but it's now (unfortunately) complicated
enough that it needs to be documented.
parent afed3545
Pipeline #575071 passed with stage
in 1 minute and 6 seconds
......@@ -97,6 +97,35 @@ If that doesn't work, please file an issue, attach the output of:
sudo G_MESSAGES_DEBUG=all /usr/libexec/power-profiles-daemon -r -v
```
Operations on Intel-based machines
----------------------------------
The "driver" for making the hardware act on the user-selected power profile on Intel
CPU-based machines is based on the [Intel P-State scaling driver](https://www.kernel.org/doc/html/v5.17/admin-guide/pm/intel_pstate.html).
It is only used if a `platform_profile` driver isn't available for the system, and the
CPU supports hardware-managed P-states (HWP). If HWP isn't supported, or the P-State
scaling driver is set to `passive` mode.
System without `platform_profile support` but with `active` P-State operation mode:
```
$ cat /sys/firmware/acpi/platform_profile_choices
cat: /sys/firmware/acpi/platform_profile_choices: No such file or directory
$ cat /sys/devices/system/cpu/intel_pstate/status
active
```
If the Intel P-State scaling driver is in `passive` mode, either because the system doesn't
support HWP, or the administator has disabled it, then the placeholder driver will be
used, and there won't be a performance mode.
Finally, if the Intel P-State scaling driver is used in `active` mode, the P-State
scaling governor will be changed to `powersave` as it is the only P-State scaling
governor that allows for the "Energy vs Performance Hints" to be taken into consideration,
ie. the only P-State scaling governor that allows power-profiles-daemon to work.
For more information, please refer to the [Intel P-State scaling driver documentation](https://www.kernel.org/doc/html/v5.17/admin-guide/pm/intel_pstate.html).
Testing
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