[PATCH] [RFC] Documentation: Add documentation for new platform_profile sysfs attribute

[Mark Pearson <markpearson@lenovo.com>]

From: Hans de Goede <hdegoede@redhat.com>

On modern systems the platform performance, temperature, fan and other
hardware related characteristics are often dynamically configurable. The
profile is often automatically adjusted to the load by somei
automatic-mechanism (which may very well live outside the kernel).

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

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

Co-developed-by: Mark Pearson <markpearson@lenovo.com>
Signed-off-by: Mark Pearson <markpearson@lenovo.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
 .../ABI/testing/sysfs-platform_profile        | 85 +++++++++++++++++++
 1 file changed, 85 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-platform_profile

diff --git a/Documentation/ABI/testing/sysfs-platform_profile b/Documentation/ABI/testing/sysfs-platform_profile
new file mode 100644
index 000000000000..37cb6275946c
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-platform_profile
@@ -0,0 +1,85 @@
+Platform-profile selection (e.g. /sys/firmware/acpi/platform_profile)
+
+On modern systems the platform performance, temperature, fan and other
+hardware related characteristics are often dynamically configurable. The
+profile is often automatically adjusted to the load by some
+automatic-mechanism (which may very well live outside the kernel).
+
+These auto platform-adjustment mechanisms often can be configured with
+one of several 'platform-profiles', with either a bias towards low-power
+consumption or towards performance (and higher power consumption and
+thermals).
+
+The purpose of the platform_profile attribute is to offer a generic sysfs
+API for selecting the platform-profile of these automatic-mechanisms.
+
+Note that this API is only for selecting the platform-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 platform-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/firmware/acpi/platform_profile_choices
+Date:		October 2020
+Contact:	Hans de Goede <hdegoede@redhat.com>
+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
+		quiet:			Offers quieter operation (lower fan
+					speed but with higher performance and
+					temperatures then seen in low-power
+		balanced:		Balance between low power consumption
+					and 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/firmware/acpi/platform_profile
+Date:		October 2020
+Contact:	Hans de Goede <hdegoede@redhat.com>
+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. 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 ai 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
+		platform-profiles. In this case writing this file will
+		override the modifications and restore the selected presets.
+
-- 
2.28.0