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 Sun, Oct 18, 2020 at 11:41 AM Hans de Goede <hdegoede@redhat.com> wrote:
>
> Hi,
>
> On 10/16/20 4:51 PM, Rafael J. Wysocki wrote:
> > 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).
>
> Currently we only have dirs under /sys/firmware:
>
> [hans@x1 ~]$ ls /sys/firmware
> acpi  dmi  efi  memmap
>
> But we do have /sys/firmware/apci/pm_profile:
>
> Documentation/ABI/stable/sysfs-acpi-pmprofile
>
> What:           /sys/firmware/acpi/pm_profile
> Date:           03-Nov-2011
> KernelVersion:  v3.2
> Contact:        linux-acpi@vger.kernel.org
> Description:    The ACPI pm_profile sysfs interface exports the platform
>                 power management (and performance) requirement expectations
>                 as provided by BIOS. The integer value is directly passed as
>                 retrieved from the FADT ACPI table.
> Values:         For possible values see ACPI specification:
>                 5.2.9 Fixed ACPI Description Table (FADT)
>                 Field: Preferred_PM_Profile
>
>                 Currently these values are defined by spec:
>                 0 Unspecified
>                 1 Desktop
>                 2 Mobile
>                 3 Workstation
>                 4 Enterprise Server
>                 ...
>
> Since all platforms which we need this for are ACPI based
> (and the involved interfaces are also all ACPI interfaces)
> how about:
>
> /sys/firmware/acpi/platform_profile
> /sys/firmware/acpi/platform_profile_choices
>
> ?
>
> I think this goes nice together with /sys/firmware/acpi/pm_profile
> although that is read-only and this is a read/write setting.
>
> Rafel, would:
>
> /sys/firmware/acpi/platform_profile
> /sys/firmware/acpi/platform_profile_choices
>
> work for you ?

Yes, it would.

Cheers!