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README: Add documentation for the AMD P-State driver

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Prajna Sariputra 2023-04-26 11:56:18 +00:00 • committed by Bastien Nocera
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README.md
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@ -134,6 +134,35 @@ ie. the only P-State scaling governor that allows HWP 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)
and the [Intel Performance and Energy Bias Hint](https://www.kernel.org/doc/html/v5.17/admin-guide/pm/intel_epb.html).
Operations on AMD-based machines
----------------------------------
The "driver" for making the hardware act on the user-selected power profile on AMD CPU-based
machines is based on the [AMD P-State scaling driver](https://www.kernel.org/doc/html/v6.3/admin-guide/pm/amd-pstate.html)
if available.
It is only used if a `platform_profile` driver isn't available for the system, the
CPU supports Collaborative Processor Performance Control (CPPC), and the AMD P-State
scaling driver is in `active` mode.
Example of a 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/amd_pstate/status
active
```
If the AMD P-State scaling driver is not loaded or is not in `active` mode, then
the placeholder driver will be used, and there won't be a performance mode.
Finally, if the AMD 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.
For more information, please refer to the [AMD P-State scaling driver documentation](https://www.kernel.org/doc/html/v6.3/admin-guide/pm/amd-pstate.html).
Testing
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