Re: [RFC] Documentation: Add documentation for new performance_profile sysfs class (Also Re: [PATCH 0/4] powercap/dtpm: Add the DTPM framework)

["Rafael J. Wysocki" <rafael@kernel.org>]

On Fri, Oct 16, 2020 at 1:11 PM Hans de Goede <hdegoede@redhat.com> wrote:
>
> <note folding the 2 threads we are having on this into one, adding every one from both threads to the Cc>
>
> Hi,
>
> On 10/14/20 5:42 PM, Rafael J. Wysocki wrote:
> > On Wed, Oct 14, 2020 at 4:06 PM Hans de Goede <hdegoede@redhat.com> wrote:
> >> On 10/14/20 3:33 PM, Rafael J. Wysocki wrote:
>
> <snip>
>
> >>> First, a common place to register a DPTF system profile seems to be
> >>> needed and, as I said above, I wouldn't expect more than one such
> >>> thing to be present in the system at any given time, so it may be
> >>> registered along with the list of supported profiles and user space
> >>> will have to understand what they mean.
> >>
> >> Mostly Ack, I would still like to have an enum for DPTF system
> >> profiles in the kernel and have a single piece of code map that
> >> enum to profile names. This enum can then be extended as
> >> necessary, but I want to avoid having one driver use
> >> "Performance" and the other "performance" or one using
> >> "performance-balanced" and the other "balanced-performance", etc.
> >>
> >> With the goal being that new drivers use existing values from
> >> the enum as much as possible, but we extend it where necessary.
> >
> > IOW, just a table of known profile names with specific indices assigned to them.
>
> Yes.
>
> > This sounds reasonable.
> >
> >>> Second, irrespective of the above, it may be useful to have a
> >>> consistent way to pass performance-vs-power preference information
> >>> from user space to different parts of the kernel so as to allow them
> >>> to adjust their operation and this could be done with a system-wide
> >>> power profile attribute IMO.
> >>
> >> I agree, which is why I tried to tackle both things in one go,
> >> but as you said doing both in 1 API is probably not the best idea.
> >> So I believe we should park this second issue for now and revisit it
> >> when we find a need for it.
> >
> > Agreed.
> >
> >> Do you have any specific userspace API in mind for the
> >> DPTF system profile selection?
> >
> > Not really.
>
> So before /sys/power/profile was mentioned, but that seems more like
> a thing which should have a set of fixed possible values, iow that is
> out of scope for this discussion.

Yes.

> Since we all seem to agree that this is something which we need
> specifically for DPTF profiles maybe just add:
>
> /sys/power/dptf_current_profile    (rw)
> /sys/power/dptf_available_profiles (ro)
>
> (which will only be visible if a dptf-profile handler
>  has been registered) ?
>
> Or more generic and thus better (in case other platforms
> later need something similar) I think, mirror the:
>
> /sys/bus/cpu/devices/cpu#/cpufreq/energy_performance_* bits
> for a system-wide energy-performance setting, so we get:
>
> /sys/power/energy_performance_preference
> /sys/power/energy_performance_available_preferences

But this is not about energy vs performance only in general, is it?

> (again only visible when applicable) ?
>
> I personally like the second option best.

But I would put it under /sys/firmware/ instead of /sys/power/ and I
would call it something like platform_profile (and
platform_profile_choices or similar).

Cheers!