linux-kernel - Re: [PATCH 0/3] ACPI: thermal: Properly support the

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Message-ID: <CAJZ5v0iEni2ie-6sCiZBfwugZ--NmJQX6=2EYKXnHwRCfpqUzQ@mail.gmail.com>
Date: Fri, 16 May 2025 15:59:39 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Armin Wolf <W_Armin@....de>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, rui.zhang@...el.com, lenb@...nel.org, 
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] ACPI: thermal: Properly support the _SCP control method

On Sat, May 3, 2025 at 12:29 AM Armin Wolf <W_Armin@....de> wrote:
>
> Am 28.04.25 um 14:34 schrieb Rafael J. Wysocki:
>
> > On Mon, Apr 28, 2025 at 2:31 PM Armin Wolf <W_Armin@....de> wrote:
> >> Am 27.04.25 um 00:52 schrieb Armin Wolf:
> >>
> >>> Am 26.04.25 um 15:12 schrieb Rafael J. Wysocki:
> >>>
> >>>> On Sat, Apr 26, 2025 at 1:20 AM Armin Wolf <W_Armin@....de> wrote:
> >>>>> Am 10.04.25 um 18:54 schrieb Armin Wolf:
> >>>>>
> >>>>>> The ACPI specification defines an interface for the operating system
> >>>>>> to change the preferred cooling mode of a given ACPI thermal zone.
> >>>>>> This interface takes the form of a special ACPI control method called
> >>>>>> _SCP (see section 11.4.13 for details) and is already supported by the
> >>>>>> ACPI thermal driver.
> >>>>>>
> >>>>>> However this support as many issues:
> >>>>>>
> >>>>>>     - the kernel advertises support for the "3.0 _SCP Extensions"
> >>>>>> yet the
> >>>>>>       ACPI thermal driver does not support those extensions. This may
> >>>>>>       confuse the ACPI firmware.
> >>>>>>
> >>>>>>     - the execution of the _SCP control method happens after the driver
> >>>>>>       retrieved the trip point values. This conflicts with the ACPI
> >>>>>>       specification:
> >>>>>>
> >>>>>>         "OSPM will automatically evaluate _ACx and _PSV objects after
> >>>>>>          executing _SCP."
> >>>>>>
> >>>>>>     - the cooling mode is hardcoded to active cooling and cannot be
> >>>>>>       changed by the user.
> >>>>>>
> >>>>>> Those issues are fixed in this patch series. In the end the user
> >>>>>> will be able to tell the ACPI firmware wether he prefers active or
> >>>>>> passive cooling. This setting will also be interesting for
> >>>>>> applications like TLP (https://linrunner.de/tlp/index.html).
> >>>>>>
> >>>>>> The whole series was tested on various devices supporting the _SCP
> >>>>>> control method and on a device without the _SCP control method and
> >>>>>> appears to work flawlessly.
> >>>>> Any updates on this? I can proof that the new interface for setting
> >>>>> the cooling mode
> >>>>> works. Additionally the first two patches fix two issues inside the
> >>>>> underlying code
> >>>>> itself, so having them inside the mainline tree would be beneficial
> >>>>> to users.
> >>>> Sure.
> >>>>
> >>>> I'm going to get to them next week, probably on Monday.
> >>> Ok, thanks.
> >>>
> >>> Armin Wolf
> >>>
> >> I am a bit ashamed of myself but i think we need to put this patch series on hold after all :(.
> >>
> >> The reason of this is that i am confused by the ACPI specification regarding _SCP:
> >>
> >>          11.1.2.1. OSPM Change of Cooling Policy
> >>
> >>          When OSPM changes the platform’s cooling policy from one cooling mode to the other, the following occurs:
> >>
> >>          1. OSPM notifies the platform of the new cooling mode by running the Set Cooling Policy (_SCP) control method in all thermal zones and invoking the OS-specific Set Cooling Policy interface to all participating devices in each thermal zone.
> >>
> >>          2. Thresholds are updated in the hardware and OSPM is notified of the change.
> >>
> >>          3. OSPM re-evaluates the active and passive cooling temperature trip points for the zone and all devices in the zone to obtain the new temperature thresholds.
> >>
> >> This section of the ACPI specification tells me that we need to evaluate the _SCP control method of all ACPI thermal zones
> >> at the same time, yet section 11.4.13. tells me that each _SCP control methods belongs to the individual thermal zone.
> >>
> >> The reason why i am concerned by this is because Windows adheres to section 11.1.2.1. and only exposes this setting
> >> as a global tunable. This might cause device manufacturers to depend on this behavior and lead to strange things
> >> should two thermal zones have different _SCP settings.
> >>
> >> I will ask the UEFI mailing list which behavior is expected by the ACPI specification. Until then i suggest that
> >> we put this patch series on hold.
> > Sure, no problem.
> >
> > Please resend it when you think it is good to go.
> >
> > Thanks!
>
> Alright, the UEFI mailing list gave no response, so i am kind of stuck.
>
> It seems that many firmware implementation only have a single cooling policy register which is set by all _SCP control methods inside the whole system.
> The reason for this seems to be that Windows treats this setting as global, but the ACPI specification seemingly does not directly mandate this.
>
> Do you think we should take the risk and allow users to control each _SCP instance manually?

No, I don't.

Doing things that are not done in Windows with ACPI objects is
generally asking for trouble unless there is a specific use case and
there is high confidence that it is actually going to work.

At least to begin with, I wouldn't do it.

> Apart from that the first two patches should be safe, so you can still pick them.

Done.

> Only the last patch needs some more work.

OK

Thanks!