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README: Update README to mention EPB

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See https://www.kernel.org/doc/html/v5.17/admin-guide/pm/intel_epb.html
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Bastien Nocera 2022-06-28 15:22:04 +02:00
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README.md
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@ -101,13 +101,14 @@ 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).
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)
or the Energy Performance Bias (EPB) feature if available.
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.
CPU supports either hardware-managed P-states (HWP) or Energy Performance Bias (EPB).
System without `platform_profile support` but with `active` P-State operation 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
@ -115,16 +116,23 @@ $ cat /sys/devices/system/cpu/intel_pstate/status
active
```
Example of a system with `EPB` support:
```
$ cat /sys/devices/system/cpu/cpu0/power/energy_perf_bias
0
```
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.
support HWP, or the administator has disabled it, and `EPB` isn't available, 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.
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).
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).
Testing
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