linuxkpi: Add device under parent, not under class

[dumbbell]

Summary

In device_add(), the function used to add the given device under its class. This is used to build a sysctl tree. We ended up with devices or "pseudo" devices (like the output connectors of a GPU). For example with an output connector:

sysctl sys.class.drm.card0-DP-1

This device should be added under its parent if it has one. With this fix, the same output connector is now:

sysctl sys.device.drmn1.card0.card0-DP-1

Sponsored by: The FreeBSD Foundation

[wulf]

I dislike this idea.
The sysctl sys.class.drm.card0-DP-1 path matches Linux while sysctl sys.device.drmn1.card0.card0-DP-1 not

[dumbbell]

Here is an example on a computer running Linux 6.18:

# ls -l /sys/class/drm
total 0
lrwxrwxrwx 1 root root    0  9 mars   2026 card0 -> ../../devices/pci0000:00/0000:00:08.1/0000:17:00.0/drm/card0
lrwxrwxrwx 1 root root    0  9 mars   2026 card0-DP-3 -> ../../devices/pci0000:00/0000:00:08.1/0000:17:00.0/drm/card0/card0-DP-3
lrwxrwxrwx 1 root root    0  9 mars   2026 card0-DP-4 -> ../../devices/pci0000:00/0000:00:08.1/0000:17:00.0/drm/card0/card0-DP-4
lrwxrwxrwx 1 root root    0  9 mars   2026 card0-DP-5 -> ../../devices/pci0000:00/0000:00:08.1/0000:17:00.0/drm/card0/card0-DP-5
lrwxrwxrwx 1 root root    0  9 mars   2026 card0-HDMI-A-3 -> ../../devices/pci0000:00/0000:00:08.1/0000:17:00.0/drm/card0/card0-HDMI-A-3
...

So the correct path on Linux is /sys/devices/pci0000:00/0000:00:08.1/0000:17:00.0/drm/card0/card0-DP-3. The difference with my proposal is that they use the PCI address (pci0000:00/0000:00:08.1/0000:17:00.0/drm), and here, we have the device name (drmn0). The other difference is that they use plural for "devices" while we use the singular "device".

The file in /sys/class/drm/... is a symlink to /sys/devices/....

I don’t find the use of /sys/class/drm logical, as the connector should be a child of the device, not a class.

Let’s collect other opinions :-)

[bz]

I have no opinion but a question: what uses this sysctl or is it purely for information?

[dumbbell]

I’m not sure anything uses it from the Ports tree as all applications and librairies will certainly be designed around sysfs.

I personnaly use it to determine which display is connected to the computer. This is useful to change my window manager configuration between the laptop’s display and my external monitor.

[bz]

I like the fact that it now should drmn<n> .

The real answer to the problem is sysfs.