Hi Julius, On 9/4/23 21:02, Julius Zint wrote: > > > On Mon, 4 Sep 2023, Thomas Weißschuh wrote: > >> +Cc Hans who ins involved with the backlight subsystem >> >> Hi Julius, >> >> today I stumbled upon a mail from Hans [0], which explains that the >> backlight subsystem is not actually a good fit (yet?) for external >> displays. >> >> It seems a new API is in the works that would better fit, but I'm not >> sure about the state of this API. Maybe Hans can clarify. >> >> This also ties back to my review question how userspace can figure out >> to which display a backlight devices applies. So far it can not. >> >> [0] https://lore.kernel.org/lkml/7f2d88de-60c5-e2ff-9b22-acba35cfdfb6@xxxxxxxxxx/ >> > > Hi Thomas, > > thanks for the hint. I will make sure to give this a proper read and > see, if it fits my use case better then the current backlight subsystem. Note the actual proposal for the new usespace API for display brightness control is here: https://lore.kernel.org/dri-devel/b61d3eeb-6213-afac-2e70-7b9791c86d2e@xxxxxxxxxx/ I have finished / stabilized the backlight code refactor which I landed in 6.1, which is a prerequisite for the above proposal. But I have not been able to make time to actually implement the above proposal; and I don't know when I will be able to make time for this. > Especially since I wasnt able to properly address your other review > comments for now. You are right that the name should align better with > the kernel module and also, that it is possible for multiple displays to > be attached. > > In its current state, this would mean that you could only control the > backlight for the first HID device (enough for me :-). > > The systemd-backlight@.service uses not only the file name, but also the > full bus path for storing/restoring backlights. I did not yet get around > to see how the desktops handle brightness control, but since the > systemd-backlight@.service already uses the name, its important to stay > the same over multiple boots. > > I would be able to get a handle on the underlying USB device and use the > serial to uniquely (and persistently) name the backlight. But it does > feel hacky doing it this way. So mutter (gnome-shell compositor library) has a similar issue when saving monitor layouts and I can tell you beforehand that monitor serial numbers by themselves are not unique enough. Some models just report 123456789 as serial and if you have a dual-monitor setup with 2 such monitors and name the backlight class device <serial>-vcp-hid or something like that you will still end up with 2 identical names. To avoid this when saving monitor layouts mutter saves both the port to which the monitor is attached (e.g. DP-1 DP-2) and the serialnumber and on startup / monitor hotplug when it checks to see if it has saved layout info for the monitor it checks the port+serialnr combination. So what I think you should do is figure out a way to map which VCP HID device maps to which drm-connector and then use the connector-name + serial-nr to generate the backlight device name. We will need the mapping the a drm-connector object anyway for the new brightness API proposal from above. Note this does NOT solve the fact that registering a new backlight class device for an external monitor on a laptop will hopelessly confuse userspace, see: https://lore.kernel.org/lkml/7f2d88de-60c5-e2ff-9b22-acba35cfdfb6@xxxxxxxxxx/ Regards, Hans
