[RFC] Documentation: Add documentation for new performance_profile sysfs class

[Hans de Goede]

On modern systems CPU/GPU/... performance is often dynamically configurable
in the form of e.g. variable clock-speeds and TPD. The performance is often
automatically adjusted to the load by some automatic-mechanism (which may
very well live outside the kernel).

These auto performance-adjustment mechanisms often can be configured with
one of several performance-profiles, with either a bias towards low-power
consumption (and cool and quiet) or towards performance (and higher power
consumption and thermals).

Introduce a new performance_profile class/sysfs API which offers a generic
API for selecting the performance-profile of these automatic-mechanisms.

Cc: Mark Pearson <markpearson@xxxxxxxxxx>
Cc: Elia Devito <eliadevito@xxxxxxxxx>
Cc: Bastien Nocera <hadess@xxxxxxxxxx>
Cc: Benjamin Berg <bberg@xxxxxxxxxx>
Cc: linux-pm@xxxxxxxxxxxxxxx
Cc: linux-acpi@xxxxxxxxxxxxxxx
Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
---
 .../testing/sysfs-class-performance_profile   | 104 ++++++++++++++++++
 1 file changed, 104 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-class-performance_profile

diff --git a/Documentation/ABI/testing/sysfs-class-performance_profile b/Documentation/ABI/testing/sysfs-class-performance_profile
new file mode 100644
index 000000000000..9c67cae39600
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-performance_profile
@@ -0,0 +1,104 @@
+Performance-profile selection (e.g. /sys/class/performance_profile/thinkpad_acpi/)
+
+On modern systems CPU/GPU/... performance is often dynamically configurable
+in the form of e.g. variable clock-speeds and TPD. The performance is often
+automatically adjusted to the load by some automatic-mechanism (which may
+very well live outside the kernel).
+
+These auto performance-adjustment mechanisms often can be configured with
+one of several performance-profiles, with either a bias towards low-power
+consumption (and cool and quiet) or towards performance (and higher power
+consumption and thermals).
+
+The purpose of the performance_profile class is to offer a generic sysfs
+API for selecting the performance-profile of these automatic-mechanisms.
+
+Note that this API is only for selecting the performance-profile, it is
+NOT a goal of this API to allow monitoring the resulting performance
+characteristics. Monitoring performance is best done with device/vendor
+specific tools such as e.g. turbostat.
+
+Specifically when selecting a high-performance profile the actual achieved
+performance may be limited by various factors such as: the heat generated by
+other components, room temperature, free air flow at the bottom of a laptop,
+etc. It is explicitly NOT a goal of this API to let userspace know about
+any sub-optimal conditions which are impeding reaching the requested
+performance level.
+
+Since numbers are a rather meaningless way to describe performance-profiles
+this API uses strings to describe the various profiles. To make sure that
+userspace gets a consistent experience when using this API this API document
+defines a fixed set of profile-names. Drivers *must* map their internal
+profile representation/names onto this fixed set.
+
+If for some reason there is no good match when mapping then a new profile-name
+may be added. Drivers which wish to introduce new profile-names must:
+1. Have very good reasons to do so.
+2. Add the new profile-name to this document, so that future drivers which also
+   have a similar problem can use the same new. Usually new profile-names will
+   be added to the "extra profile-names" section of this document. But in some
+   cases the set of standard profile-names may be extended.
+
+What:		/sys/class/performance_profile/<device>/available_profiles
+Date:		October 2020
+Contact:	Hans de Goede <hdegoede@xxxxxxxxxx>
+Description:
+		Reading this file gives a space separated list of profiles
+		supported for this device.
+
+		Drivers must use the following standard profile-names whenever
+		possible:
+
+		low-power:		Emphasises low power consumption
+					(and also cool and quiet)
+		balanced-low-power:	Balances between low power consumption
+					and performance with a slight bias
+					towards low power
+		balanced:		Balance between low power consumption
+					and performance
+		balanced-performance:	Balances between performance and low
+					power consumption with a slight bias
+					towards performance
+		performance:		Emphasises performance (and may lead to
+					higher temperatures and fan speeds)
+
+		Userspace may expect drivers to offer at least several of these
+		standard profile-names! If none of the above are a good match
+		for some of the drivers profiles, then drivers may use one of
+		these extra profile-names:
+		<reserved for future use>
+
+What:		/sys/class/performance_profile/<device>/current_profile
+Date:		October 2020
+Contact:	Hans de Goede <hdegoede@xxxxxxxxxx>
+Description:
+		Reading this file gives the current selected profile for this
+		device. Writing this file with one of the strings from
+		available_profiles changes the profile to the new value.
+
+		Reading this file may also return "custom". This is intended for
+		drivers which have and export multiple knobs influencing
+		performance. Such drivers may very well still want to offer a
+		set of profiles for easy of use and to be able to offer a
+		consistent standard API (this API) to userspace for configuring
+		their performance. The "custom" value is intended for when a
+		user has directly configured the knobs (through e.g. some
+		advanced control-panel for a GPU) and the knob values do not
+		match any of the presets represented by the
+		performance-profiles. In this case writing this file will
+		override the modifications and restore the selected presets.
+
+What:		/sys/class/performance_profile/<device>/type
+Date:		October 2020
+Contact:	Hans de Goede <hdegoede@xxxxxxxxxx>
+Description:
+		Performance-profiles may be system-wide, or for a specific
+		device (e.g. CPU / GPU). System-wide profiles are typically
+		used on devices where where a single cooling solution is
+		shared between all components, such as laptops and NUCs.
+
+		Reading this file indicates the type of the device for which
+		the thermal-profile is being configured.
+
+		Valid values: "system"
+		Reserved for future use values: "cpu", "gpu"
-- 
2.28.0