Documentation: Add documentation for new platform_profile sysfs attribute

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).

Introduce a new platform_profile sysfs API which offers a generic API for
selecting the performance-profile of these automatic-mechanisms.
Co-developed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mark Pearson <markpearson@lenovo.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/ABI/testing/sysfs-platform_profile

+What:		/sys/firmware/acpi/platform_profile_choices
+Date:		October 2020
+Contact:	Hans de Goede <hdegoede@redhat.com>
+Description:	This file contains a space-separated list of profiles supported for this device.
+
+		Drivers must use the following standard profile-names:
+
+		============	============================================
+		low-power	Low power consumption
+		cool		Cooler operation
+		quiet		Quieter operation
+		balanced	Balance between low power consumption and performance
+		performance	High performance operation
+		============	============================================
+
+		Userspace may expect drivers to offer more than one of these
+		standard profile names.
+
+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
+		platform_profile_choices changes the profile to the new value.

Documentation/userspace-api/index.rst

@@ -24,6 +24,7 @@ place where this information is gathered.
    ioctl/index
    iommu
    media/index
+   sysfs-platform_profile
 
 .. only::  subproject and html
 

Documentation/userspace-api/sysfs-platform_profile.rst

+=====================================================================
+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
+platform configuration is often automatically adjusted to the current
+conditions 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
+operation or towards performance.
+
+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 on their own cannot represent the multiple variables that a
+profile will adjust (power consumption, heat generation, etc) this API
+uses strings to describe the various profiles. To make sure that userspace
+gets a consistent experience the sysfs-platform_profile ABI document defines
+a fixed set of profile names. Drivers *must* map their internal profile
+representation onto this fixed set.
+
+If 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. Explain why the existing profile names canot be used.
+ 2. Add the new profile name, along with a clear description of the
+    expected behaviour, to the sysfs-platform_profile ABI documentation.