linux-kernel - parent device of thermal cooling device

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Date:   Wed, 27 Apr 2022 01:08:05 +0200
From:   Armin Wolf <W_Armin@....de>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Amit Kucheria <amitk@...nel.org>, Zhang Rui <rui.zhang@...el.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: parent device of thermal cooling device

Hello,

i am currently working on integrating an hwmon driver (dell_smm_hwmon) into the thermal subsystem.
While doing that, i noticed that when registering cooling devices, the thermal subsystem
does not allow passing a parent device, so there is no "device/" symlink being created.

This creates problems for example when a thermal driver is instantiated multiple times while
creating cooling devices with the same type. Without the "device/" symlink, userspace is
unable to tell which cooling device belongs to which device.

The acpi fan driver is already inserting a symlink between the cooling device and the ACPI device
to avoid such issues, but such a symlink could be created automatically by allowing the driver
to pass a parent device when registering with the thermal subsystem.

Two approaches are possible:

1. Pass a separate device struct as parent device to __thermal_cooling_device_register().

2. Replace the device tree node argument with a parent device and use the device tree node
from this device.

Which approach would be the more feasible one?

Armin Wolf