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

[Limonciello, Mario]

> 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.
> 

If introducing an API for this - let me ask the question, why even let each
driver offer a class interface and userspace need to change "each" driver's
performance setting?

I would think that you could just offer something kernel-wide like
/sys/power/performance-profile

Userspace can read and write to a single file.  All drivers can get notified
on this sysfs file changing.

The systems that react in firmware (such as the two that prompted
this discussion) can change at that time.  It leaves the possibility for a
more open kernel implementation that can do the same thing though too by
directly modifying device registers instead of ACPI devices.

> 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).

Are you intending to word this specifically to cover both firmware and userspace
implementations?  Or were you really meaning firmware implementations?

> +
> +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.

Another thought that comes to mind (which is completely separate from my previous
idea):

Why not make this register to firmware-attributes class as being discussed in the
new Dell driver?

It seems like it could easily be read as:
/sys/class/firmware-attributes/thinkpad-foo/attributes/PerformanceProfile/current_value
/sys/class/firmware-attributes/thinkpad-foo/attributes/PerformanceProfile/possible_values


> +
> +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