Page MenuHomeSolus

Light doesn't seem to work
Open, LowPublic


Light, an xbacklight replacement for i3 as the description says, isn't working. Manpage shows all the info, but no command is working.

I have an Acer 5755 with i7-2670QM. X11 works just fine even with keyboard shortcuts and manual xbacklight command, but not on wayland. This is one of the larger problems I'm facing on wayland, and I'm not sure how to combine these into a single issue. Help regarding that, too is appreciated.

Event Timeline

whatever created this task.Aug 3 2018, 6:12 PM
sunnyflunk added a subscriber: sunnyflunk.

Most X apps like this won't work on Wayland.

[Using X speak] The wayland server (typically the WM you are running) typically controls everything the X server used to. For an external app to make any adjustment to the wayland server it requires support built into the WM to allow external apps to do so (which is likely highly restricted to approved apps), and the app needs to be built to support the WM. So as far as I see it's working as intended, wayland WM's need to include all these features now, and users need to select wayland WM's that include the features they want (or use X).

Except Light isn't an X application. It communicates directly with the kernel to modify values in /sys/class/backlight/ to change brightness. I believe other utilities such as acpilight and brightnessctl do the same. Also, light is one of the recommended applications for brightness control as a replacement for xbacklight on sway wiki, which is why I installed it at all.

Lets be clear and compare apples with apples:

Does light work on X? (that's a pretty important piece of information). It seems like you are using a completely different program (which then provides no useful information about the light issue). The fact it's a rundep of i3 suggests that it should and if it doesn't work on X, there's no reason it would work on wayland.

If it doesn't work on either X or Wayland, then it's more likely a hardware/kernel issue where you need a kernel cmdline override to adjust acpi (or other setting). Then referring to X or wayland is irrelevant.

whatever added a comment.EditedAug 4 2018, 1:26 AM

Huh. You're right. It does NOT work in X either. So, what can I try next?

Edit : I also can't change values in /sys/class/backlight/ manually. Even with sudo, I'm getting permission denied.

@kyrios123 I recall seeing you writing a paragraph about acpi_osi= somewhere recently, but I can't recall where.

In short, acpi_osi= and acpi_backlight= seem to be most problematic on Linux to need overriding kernel parameters. It can only be tested on the actual hardware though trial and error most likely. Maybe needs a quick write up how to do this.

Fair enough, we'll wait for kyrios to comment. In the meanwhile, what can I do to change brightness on wayland? echo number >> /sys/class/backlight/intel_backlight/brightness isn't working, so I'm having to switch to plasma every time I want to change brightness.

sunnyflunk added a comment.EditedAug 4 2018, 7:10 AM

In the meanwhile, what can I do to change brightness on wayland?

Don't use wayland if you want to be able to change brightness...

If it's from me, it was most likely on the forums and it will be hard to find it back.
Or perhaps it was this task from ermo ?

Out of interest, does light work on the plasma X session?

Out of interest, does light work on the plasma X session?


DataDrake triaged this task as Low priority.Oct 16 2018, 11:39 PM
DataDrake moved this task from Backlog to Package Fixes on the Software board.