Ubuntu – Keyboard backlight turns on after lock screen/display on

15.10dellkeyboardkeyboard-backlightlock-screen

I've just upgraded Ubuntu (15.04 to 15.10) and now I'm running into a weird behavior: whenever I lock the screen (using any of Ctrl+Alt+L, Win+L or just clicking on "Lock" in the taskbar menu), the keyboard backlight turns on, even though I always keep it off.

I have no idea where to begin investigating this. It didn't happen in Ubuntu 15.04, and it doesn't seem other people have run into this.

My laptop is a Dell Inspiron, if it helps.

Update: I later understood that this problem happens whenever the display turns on, which happens right after I lock the screen because of a bug. So, these are related events, but the actual cause is the screen turning on.

Best Answer

So, the Web is truly an amazing thing! After much research and diving into many forum links, I managed to better understand the problem, to find out what program was responsible for it and, finally, to disable this behavior for good. I thought I'd write down my steps, in case other people run into the same issue.

Step 1. Monitor the system to know if/when the keyboard backlight is being turned on

The first thing is to know that the current status of the keyboard backlight can be checked by reading the file /sys/devices/platform/dell-laptop/leds/dell\:\:kbd_backlight/brightness. So, we would like to be notified of any changes to this file and, ideally, what process invoked the change.

This can be done using fanotify. I used the example program that is provided in the man page and added a few modifications* so that I could monitor changes to the brightness file. Turns out that the process responsible for the modification was /usr/lib/upower/upowerd, which is a daemon process for UPower.

Step 2. Find out why UPower is turning the backlight on (fail)

This was the fuzziest part of the investigation, and I still can't tell why this happens. I found a person with a very similar problem but with no solution, so I started to see if I could just disable UPower completely (and what would the consequences be) when I found this link. Turns out UPower has a configuration file! That's better.

Step 3. Stop UPower from turning on the keyboard backlight

Finally, I edited the aforementioned config file, located at /etc/dbus-1/system.d/org.freedesktop.UPower.conf, and changed <allow> to <deny> in the following line:

<allow send_destination="org.freedesktop.UPower"
       send_interface="org.freedesktop.UPower.KbdBacklight"/>

I then restarted dbus with sudo service dbus restart (use systemctl restart dbus.service on Ubuntu 16.04 and later) and proceeded to lock my screen to test the result and... It worked!! Well, sorta. The login screen wouldn't show up, so I couldn't log back in and had to force a restart. But then everything worked great!

And that's it! I'm still curious what would be the right way to do this. If you know, leave a comment, please!

* For instance, the example program was only set up to monitor FAN_OPEN_PERMand FAN_CLOSE_WRITE events, but in this it was an FAN_MODIFY event.

Related Solutions

Ubuntu – How to reduce delay before locking after pressing lock button

I searched a lot to find anything (configuration) that may be related to this lag but to luck. It seems that there was a known bug [old]. anyway we still can use other tools to achieve what we want unless that bug is effecting you which you can use the above link instruction to get rid of it..

First use these commands to lock your screen, see which one of them is work better for you and which one you prefer to use.

$ dm-tool lock
$ gnome-screensaver-command -a  # starts screen saver

Now open system settings go to keyboard section and shortcuts tabs.

Now you can select custom shortcuts and asign something Ctrl+Alt+l to a command like dm-tool lock; Cause by default we can't use Super+l.

However if you insist to use Super+l to run your preferred command, set it as your key-binding and follow along to make it work.

To disable unity's Super+l key-binding, run:

dconf write /org/compiz/profiles/unity/plugins/unityshell/lock-screen "''"

To restore default behavior:

dconf reset /org/compiz/profiles/unity/plugins/unityshell/lock-screen

or install compiz setting manager:

$ sudo apt install compizconfig-settings-manager

then run it:

$ ccsm

Go to ubuntu unity plugin section. on line two, Key to lock the screen, click on the button <Super>l, uncheck the Enabled check-box click on ok.

Ubuntu – Screen lock shuts down all applications in 17.10

I use a 5 minute screen blank on 16.04, 17.10 and 18.04 beta, and it works as expected for me. You might also want to check launchpad bug reports, but I suspect that something else is causing problems with your system than the lock screen and blanking alone. For example, I wonder if some extension is crashing the gnome-shell (or something equiv if you are using a different desktop)?

You might check logs e.g. journalctl -b0 to look for suspicious errors in and around the time of the blank or the unlock. journalctl is better than dmesg for this because it provides human readable dates for logged events.