Message-ID: <e360b9b3-ada4-4cd1-8971-097484cf3f5f@gmx.de>
Date: Thu, 27 Nov 2025 21:06:44 +0100
From: Armin Wolf <W_Armin@....de>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Daniel Lezcano <daniel.lezcano@...aro.org>,
 Zhang Rui <rui.zhang@...el.com>, Lukasz Luba <lukasz.luba@....com>,
 Len Brown <lenb@...nel.org>, Jonathan Corbet <corbet@....net>,
 Ido Schimmel <idosch@...dia.com>, Petr Machata <petrm@...dia.com>,
 linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
 etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
 linux-tegra@...r.kernel.org, linux-acpi@...r.kernel.org,
 linux-doc@...r.kernel.org, netdev@...r.kernel.org,
 linux-wireless@...r.kernel.org, ath10k@...ts.infradead.org,
 ath11k@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org,
 linux-mediatek@...ts.infradead.org, platform-driver-x86@...r.kernel.org,
 linux-pci@...r.kernel.org, imx@...ts.linux.dev,
 linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH RFC RESEND 0/8] thermal: core: Allow setting the parent
 device of thermal zone/cooling devices

Am 27.11.25 um 18:41 schrieb Rafael J. Wysocki:

> On Sat, Nov 22, 2025 at 3:18 PM Armin Wolf <W_Armin@....de> wrote:
>> Am 21.11.25 um 21:35 schrieb Rafael J. Wysocki:
>>
>>> On Thu, Nov 20, 2025 at 4:41 AM Armin Wolf <W_Armin@....de> wrote:
>>>> Drivers registering thermal zone/cooling devices are currently unable
>>>> to tell the thermal core what parent device the new thermal zone/
>>>> cooling device should have, potentially causing issues with suspend
>>>> ordering
>>> This is one potential class of problems that may arise, but I would
>>> like to see a real example of this.
>>>
>>> As it stands today, thermal_class has no PM callbacks, so there are no
>>> callback execution ordering issues with devices in that class and what
>>> other suspend/resume ordering issues are there?
>> Correct, that is why i said "potentially".
>>
>>> Also, the suspend and resume of thermal zones is handled via PM
>>> notifiers.  Is there a problem with this?
>> The problem with PM notifiers is that thermal zones stop working even before
>> user space is frozen. Freezing user space might take a lot of time, so having
>> no thermal management during this period is less than ideal.
> This can be addressed by doing thermal zone suspend after freezing
> tasks and before starting to suspend devices.  Accordingly, thermal
> zones could be resumed after resuming devices and before thawing
> tasks.  That should not be an overly complex change to make.

AFAIK this is only possible by using dev_pm_ops, the PM notifier is triggered before
tasks are frozen during suspend and after they are thawed during resume.

Using dev_pm_ops would also ensure that thermal zone devices are resumed after their
parent devices, so no additional changes inside the pm core would be needed.

>> This problem would not occur when using dev_pm_ops, as thermal zones would be
>> suspended after user space has been frozen successfully. Additionally, when using
>> dev_pm_ops we can get rid of thermal_pm_suspended, as the device core already mandates
>> that no new devices (including thermal zones and cooling devices) be registered during
>> a suspend/resume cycle.
>>
>> Replacing the PM notifiers with dev_pm_ops would of course be a optimization with
>> its own patch series.
> Honestly, I don't see much benefit from using dev_pm_ops for thermal
> zone devices and cooling devices.  Moreover, I actually think that
> they could be "no PM" devices that are not even put on the
> suspend-resume device list.  Technically, they are just interfaces on
> top of some other devices allowing the user space to interact with the
> latter and combining different pieces described by the platform
> firmware.  They by themselves have no PM capabilities.

Correct, thermal zone devices are virtual devices representing thermal management
aspects of the underlying parent device. This however does not mean that thermal zone
devices have no PM capabilities, because they contain state. Some part of this state
(namely TZ_STATE_FLAG_SUSPENDED and TZ_STATE_FLAG_RESUMING) is affected by power management,
so we should tell the device core about this by using dev_pm_ops instead of the PM notifier.

>>>> and making it impossible for user space applications to
>>>> associate a given thermal zone device with its parent device.
>>> Why does user space need to know the parent of a given cooling device
>>> or thermal zone?
>> Lets say that we have two thermal zones registered by two instances of the
>> Intel Wifi driver. User space is currently unable to find out which thermal zone
>> belongs to which Wifi adapter, as both thermal zones have the (nearly) same type string ("iwlwifi[0-X]").
> But the "belong" part is not quite well defined here.  I think that
> what user space needs to know is what devices are located in a given
> thermal zone, isn't it?  Knowing the parent doesn't necessarily
> address this.

The device exposing a given thermal zone device is not always a member of the thermal zone itself.
In case of the Intel Wifi adapters, the individual Wifi adapters are indeed members of the thermal zone
associated with their thermal zone device. But thermal zones created thru a system management controller
for example might only cover devices like the CPUs and GPUs, not the system management controller device itself.

The parent device of a child device is the upstream device of the child device. The connection between parent
and child can be physical (SMBus controller (parent) -> i2c device (child)) or purely logical
(PCI device (parent) -> thermal zone device (child)). There exists a parent-child dependency between a parent
and a child device (the child device cannot function without its parent being operational), and user space
might want to be able to discover such dependencies.

>> This problem would be solved once we populate the parent device pointer inside the thermal zone
>> device, as user space can simply look at the "device" symlink to determine the parent device behind
>> a given thermal zone device.
> I'm not convinced about this.
>
>> Additionally, being able to access the acpi_handle of the parent device will be necessary for the
>> ACPI thermal zone driver to support cooling devices other than ACPI fans and ACPI processors.
> I guess by the "parent" you mean the device represented in the ACPI
> namespace by a ThermalZone object, right?  But this is not the same as
> the "parent" in the Wifi driver context, is it?

In the context of a ACPI ThermalZone, the parent device of the thermal cooling device would currently
be the ACPI device bound to the "thermal" ACPI driver. In the context of the Intel Wifi card, the parent
device would be PCI device bound to the corresponding Intel Wifi driver.

I think you misunderstood what kind of parent device i was referring to. You likely though that i was referring
to the parent device of the ACPI ThermalZone, right? That however is not the case , with "parent device" i was
referring to the device responsible for creating a given struct thermal_zone_device instance.

>>>> This patch series aims to fix this issue by extending the functions
>>>> used to register thermal zone/cooling devices to also accept a parent
>>>> device pointer. The first six patches convert all functions used for
>>>> registering cooling devices, while the functions used for registering
>>>> thermal zone devices are converted by the remaining two patches.
>>>>
>>>> I tested this series on various devices containing (among others):
>>>> - ACPI thermal zones
>>>> - ACPI processor devices
>>>> - PCIe cooling devices
>>>> - Intel Wifi card
>>>> - Intel powerclamp
>>>> - Intel TCC cooling
>>> What exactly did you do to test it?
>> I tested:
>> - the thermal zone temperature readout
>> - correctness of the new sysfs links
>> - suspend/resume
>>
>> I also verified that ACPI thermal zones still bind with the ACPI fans.
> I see, thanks.
>
>>>> I also compile-tested the remaining affected drivers, however i would
>>>> still be happy if the relevant maintainers (especially those of the
>>>> mellanox ethernet switch driver) could take a quick glance at the
>>>> code and verify that i am using the correct device as the parent
>>>> device.
>>> I think that the above paragraph is not relevant any more?
>> You are right, however i originally meant to CC the mellanox maintainers as
>> i was a bit unsure about the changes i made to their driver. I will rework
>> this section in the next revision and CC the mellanox maintainers.
>>
>>>> This work is also necessary for extending the ACPI thermal zone driver
>>>> to support the _TZD ACPI object in the future.
>>> I'm still unsure why _TZD support requires the ability to set a
>>> thermal zone parent device.
>> _TZD allows the ACPI thermal zone to bind to cooling devices other than ACPI fans
>> and ACPI processors, like ACPI batteries.
> No, it is not for cooling devices if my reading of the specification
> is correct.  It says:
>
> "_TZD (Thermal Zone Devices)
>
> This optional object evaluates to a package of device names. Each name
> corresponds to a device in the ACPI namespace that is associated with
> the thermal zone. The temperature reported by the thermal zone is
> roughly correspondent to that of each of the devices."
>
> And then
>
> "The list of devices returned by the control method need not be a
> complete and absolute list of devices affected by the thermal zone.
> However, the package should at least contain the devices that would
> uniquely identify where this thermal zone is located in the machine.
> For example, a thermal zone in a docking station should include a
> device in the docking station, a thermal zone for the CD-ROM bay,
> should include the CD-ROM."
>
> So IIUC this is a list of devices allowing the location of the thermal
> zone to be figured out.  There's nothing about cooling in this
> definition.

Using _TZD to figure out the location of a given thermal zone is another usage
of this ACPI control method, but lets take a look at section 11.6:

- If _PSV is defined then either the _PSL or _TZD objects must exist. The _PSL and _TZD objects may both exist.
- If _PSV is defined and _PSL is not defined then at least one device in thermal zone, as indicated by either the
   _TZD device list or devices’ _TZM objects, must support device performance states.

So according to my understanding, _TZD can also be used to discover additional cooling devices used for passive cooling.
This makes sense as _PSL is defined to only contain processor objects (see section 11.4.10), so _TZD can act like an
extension of _PSL for things like ACPI control method batteries (see 10.2.2.12).

Microsoft also follows this approach (see https://learn.microsoft.com/en-us/windows-hardware/design/device-experiences/design-guide
section "Thermally managed devices" paragraph "Processor aggregator").

>> This however will currently not work as
>> the ACPI thermal zone driver uses the private drvdata of the cooling device to
>> determine if said cooling device should bind. This only works for ACPI fans and
>> processors due to the fact that those drivers store a ACPI device pointer inside
>> drvdata, something the ACPI thermal zone expects.
> I'm not sure I understand the above.
>
> There is a list of ACPI device handles per trip point, as returned by
> either _PSL or _ALx.  Devices whose handles are in that list will be
> bound to the thermal zone, so long as there are struct acpi_device
> objects representing them which is verified with the help of the
> devdata field in struct thermal_cooling_device.

AFAIK devdata is meant to be used by the thermal zone device callbacks to access the state
container struct of the associated device driver instance. Assuming that a given device driver
will populate devdata with a pointer to is ACPI companion device is an implementation-specific
detail that does not apply to all cooling device implementations. It just so happens that the
ACPI processor and fan driver do this, likely because they where designed specifically to work
with the ACPI thermal zone driver.

The documentation of thermal_cooling_device_register() even describes devdata as "device private data", so any meaning of devdata purely depends on the 
given device driver.

> IOW, cooling device drivers that create struct thermal_cooling_device
> objects representing them are expected to set devdata in those objects
> to point to struct acpi_device objects corresponding to their ACPI
> handles, but in principle acpi_thermal_should_bind_cdev() might as
> well just use the handles themselves.  It just needs to know that
> there is a cooling driver on the other side of the ACPI handle.
>
> The point is that a cooling device to be bound to an ACPI thermal zone
> needs an ACPI handle in the first place to be listed in _PSL or _ALx.

Correct, i merely change the way the ACPI thermal zone driver retrieves the
ACPI handle associated with a given cooling device.

>> As we cannot require all cooling devices to store an ACPI device pointer inside
>> their drvdata field in order to support ACPI,
> Cooling devices don't store ACPI device pointers in struct
> thermal_cooling_device objects, ACPI cooling drivers do, and there are
> two reasons to do that: (1) to associate a given struct
> thermal_cooling_device with an ACPI handle and (2) to let
> acpi_thermal_should_bind_cdev() know that the cooling device is
> present and functional.
>
> This can be changed to store an ACPI handle in struct
> thermal_cooling_device and acpi_thermal_should_bind_cdev() may just
> verify that the device is there by itself.

I can of course extend thermal_cooling_device_register() to accept a fwnode_handle that
can be used for both ACPI and OF based cooling device identification, if this is what you
prefer.

This patch series would then turn into a cleanup series, focusing on properly adding
thermal zone devices and cooling devices into the global device hierarchy.

>> we must use a more generic approach.
> I'm not sure what use case you are talking about.
>
> Surely, devices with no representation in the ACPI namespace cannot be
> bound to ACPI thermal zones.  For devices that have a representation
> in the ACPI namespace, storing an ACPI handle in devdata should not be
> a problem.

See my above explanations for details, drvdata is defined to hold device private data,
nothing more.

>> I was thinking about using the acpi_handle of the parent device instead of messing
>> with the drvdata field, but this only works if the parent device pointer of the
>> cooling device is populated.
>>
>> (Cooling devices without a parent device would then be ignored by the ACPI thermal
>> zone driver, as such cooling devices cannot be linked to ACPI).
> It can be arranged this way, but what's the practical difference?
> Anyone who creates a struct thermal_cooling_device and can set its
> parent pointer to a device with an ACPI companion, may as well set its
> devdata to point to that companion directly - or to its ACPI handle if
> that's preferred.

Yes, but this would require explicit support for ACPI in every driver that registers cooling devices.
Using the parent device to retrieve the acpi_handle or allowing all drivers to just submit a fwnode_handle
of their choice when creating a cooling device will fix this.

Thanks,
Armin Wolf

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