Bug 66367

Summary:	Provide handle for ACPI to change backlight brightness, similiar to other power-related ACPI events
Product:	systemd	Reporter:	Peter Weber <peter.weber>
Component:	general	Assignee:	systemd-bugs
Status:	RESOLVED WONTFIX	QA Contact:	systemd-bugs
Severity:	enhancement
Priority:	medium	CC:	i.gnatenko.brain
Version:	unspecified
Hardware:	All
OS:	Linux (All)
Whiteboard:
i915 platform:		i915 features:

Description Peter Weber 2013-06-29 15:34:09 UTC

Hello!

Proposal:
Add a new handle named like "HandleBrightness" to Systemd and let it take care of the brightness-events, which the kernel emits and signals to userspace. Similiar to "HandleLidSwitch" or "HandleSuspendKey" and configureable via "/etc/systemd/logind.conf". If a desktop-environment like GNOME, KDE or XFCE is able to handle this itself, it should "inhibit" the handle.

I would avoid the postfix "Keys" because such an event could be trigged by other things, like a light-sensor or something similiar.

Rationale:
Currently the handling of events triggered through the backlight-keys is weird on GNU/Linux. If a key-press emits an event, the Linux-Kernel sends the event to the userspace. At the same time, it is possible that the driver ACPI_BACKLIGHT, inside the kernel, reacts on the event and changes the backlight-brightness itself.

This is likely to change in future, instead the events will be only passed to userspace.
http://lists.freedesktop.org/archives/intel-gfx/2013-March/026016.html
Patches:
https://patchwork.kernel.org/patch/2695411/
https://patchwork.kernel.org/patch/2695391/
https://patchwork.kernel.org/patch/2695401/

As a consequence the brightness-keys won't work on a terminal (TTY) or a plain window-manager based on X11 or Wayland:
https://bugzilla.kernel.org/show_bug.cgi?id=51231#c107

Benefit:
This allows for changing the backlight-brigthness out-of-the-box. Especially on a terminal (TTY), but also on any other kind of environmnet on top of X11 or Wayland (i3wm, fluxbox, window-maker...$YOUR_SETUP_HERE).

Thank you
Peter

Comment 1 Peter Weber 2013-06-29 15:45:03 UTC

I've to apologize. Instead of the link to freedesktop:
https://lkml.org/lkml/2013/6/9/161

Comment 2 Igor Gnatenko 2013-07-01 08:32:23 UTC

(In reply to comment #1)
> I've to apologize. Instead of the link to freedesktop:
> https://lkml.org/lkml/2013/6/9/161
I think this method in systemd is better.

Comment 3 Lennart Poettering 2013-09-12 18:04:02 UTC

Hmm, I am not convinced we really want this in logind. In contrast to handling of lid switch/power button/suspend button this isn't really part of the system lifecycle. It's an auxiliary thing really... I am tempted to say that gnome-settings-daemon is probably a better place for handling this...

Comment 4 Peter Weber 2013-09-14 18:04:52 UTC

Thanks for your response. I thought about HandleLitSwitch or HandleSuspendKey more in a way of basic integration of power-management, instead of a integration into the system-lifecycle. But it's obvious for me now and makes sense! You're right with this.

Well. Handling the brightness in Systemd looked like an straight solution, till now. The problem with gnome-settings-daemon is, that it requires a desktop-environment at all with the complete stoftware stack from DBUS to X11/Wayland. The changes to the kernel breaks "brigthness-keys" for users of plain TTYs and non-desktop-environements. Also gnome-settings-daemon ignores the events from the keys, if the users operates currently on a TTY and not in GNOME.

Maybe writing and shell-script (with an alias for it) which modifies the content in the responsible files in /sys/... to change the brigthness would be a sufficient solution for me.

Comment 5 Bastien Nocera 2013-10-11 07:20:00 UTC

The brightness changes should be per-output, and there's no way that you can adequately do that in the console, without a display manager of some sort, that would direct the backlight changes to the correct output.

Xorg drivers mostly use libbacklight[1] nowadays for that purpose, ensuring that they do not trample on each other (there might be multiple brightness controls for each output, and none for others).

[1]: http://cgit.freedesktop.org/libbacklight/

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