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Subject: Re: [RFC] Documentation: Add documentation for new
 performance_profile sysfs class (Also Re: [PATCH 0/4] powercap/dtpm: Add the
 DTPM framework)
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     Daniel Lezcano <daniel.lezcano@linaro.org>,
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From:   Hans de Goede <hdegoede@redhat.com>
Message-ID: <943531a7-74d6-7c7f-67bc-2645b3ba7b8a@redhat.com>
Date:   Fri, 16 Oct 2020 13:10:54 +0200
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<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.

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

(again only visible when applicable) ?

I personally like the second option best.

Regards,

Hans