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pjp
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Joined: 16 Apr 2002
Posts: 20067
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 Posted: Tue Jun 21, 2022 4:32 am Post subject: [solved] How does / did x11-apps/xbacklight change settings? |
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Neither xbacklight nor xrandr have setuid/setgid bits set. Other than being in the video group, no special permissions appeared necessary.
| Code: | $ xrandr --listactivemonitors
Monitors: 1
0: +*eDP-1 1920/309x1080/174+0+0 eDP-1
$ find /sys/devices/ -name "*eDP*"
/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1
$ cat /sys/devices/pci0000\:00/0000\:00\:02.0/drm/card0/dev
226:0
$ ls -l /dev/dri/card0
crw-rw---- 1 root video 226, 0 Jun 19 06:06 /dev/dri/card0
$ echo 500 > /sys/class/backlight/intel_backlight/brightness
bash: /sys/class/backlight/intel_backlight/brightness: Permission denied |
I can't modify the brightness setting as a normal user.
Other than needing to be root, interacting with the /sys/class files seems easy enough, so I was considering writing my own utility to do so.
That's how acpilight appears to function. The ebuild for acpilight requires acct-group/video and mentions the need to be a member of that group.
However, I don't see anywhere indicating how it can modify /sys/class files which are group owned by root, not video (at least the ones I've looked at), so acpilight's xbacklight command having group video membership doesn't seem to be the answer.
What piece am I missing that allows either x11-apps/xbacklight or acpilight to make those changes?
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Last edited by pjp on Tue Jun 21, 2022 2:10 pm; edited 1 time in total |
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Anon-E-moose
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Joined: 23 May 2008
Posts: 6098
Location: Dallas area
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| Posted: Tue Jun 21, 2022 9:24 am Post subject: |
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| Code: | $ grep DESC /usr/portage/x11-apps/xbacklight/xbacklight-1.2.3.ebuild
DESCRIPTION="Sets backlight level using the RandR 1.2 BACKLIGHT output property" |
https://wiki.archlinux.org/title/backlight has some info that might be useful
Acpilight should just work though.
What are the permissions on /sys/class/backlight/*
And
It's easy enough to change permissions on those files when booting up, I like to use /etc/local.d/baselayout1.start for things like that.
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pjp
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| Posted: Tue Jun 21, 2022 2:10 pm Post subject: |
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I noticed that description, which is why I looked at xrandr. It of course says to use xbacklight :)
I guess xrandr is probably implementing "RandR 1.2" and I'd have to look at its code.
Changing permissions was the clue, although it seems like a hackish solution. acpilight has a udev rule which does that as well as changing group ownership. I was anticipating whatever it implemented would be more "involved" that reading and writing those files.
Thanks!
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Anon-E-moose
Watchman
Joined: 23 May 2008
Posts: 6098
Location: Dallas area
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| Posted: Tue Jun 21, 2022 4:13 pm Post subject: |
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There's not much more than reading/writing 
I don't have a problem using the /sys subsystem, it's simply a raw interface to many things in the kernel/modules.
xrandr --help gives
| Code: | ...
--output <output>
--auto
--mode <mode>
--preferred
--pos <x>x<y>
--rate <rate> or --refresh <rate>
--reflect normal,x,y,xy
--rotate normal,inverted,left,right
--left-of <output>
--right-of <output>
--above <output>
--below <output>
--same-as <output>
--set <property> <value>
--scale <x>[x<y>]
--scale-from <w>x<h>
--transform <a>,<b>,<c>,<d>,<e>,<f>,<g>,<h>,<i>
--filter nearest,bilinear
--off
--crtc <crtc>
--panning <w>x<h>[+<x>+<y>[/<track:w>x<h>+<x>+<y>[/<border:l>/<t>/<r>/<b>]]]
--gamma <r>[:<g>:<b>]
--brightness <value>
--primary |
So brightness should work, using the latest xrandr 1.5.1, but I would favor just modifying permissions.
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pjp
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| Posted: Tue Jun 21, 2022 5:01 pm Post subject: |
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Maybe I'm lucky in that I have 0 udev rules, but it didn't even occur to me that something relevant would have been there. I guess it has been long enough that I mostly forgot why they exist.
| Quote: | --brightness brightness
Multiply the gamma values on the crtc currently attached to the output to specified floating value. Useful for overly bright or
overly dim outputs. However, this is a software only modification, if your hardware has support to actually change the bright‐
ness, you will probably prefer to use xbacklight. |
It wasn't readily obvious what crtc was and the mentioned differentiation between software and hardware seemed to indicate there was a difference between using xrandr's brightness option and that of the non-acpilight xbacklight.
If using /sys is the "correct" way to do it, then I'll use that method, but it still seems like a hack *shrug*. Changing ownership and permissions is a standard Unix way of doing things, so at least there's that, which is nice.
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GDH-gentoo
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Joined: 20 Jul 2019
Posts: 1530
Location: South America
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| Posted: Wed Jun 22, 2022 3:12 am Post subject: Re: |
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| pjp wrote: | Neither xbacklight nor xrandr have setuid/setgid bits set. [...]
What piece am I missing that allows either x11-apps/xbacklight or acpilight to make those changes? |
I don't know about acpilight, but xbacklight doesn't actually make the changes itself. It is an X client, and asks the X server to make them.
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| Ionen wrote: | | As a packager I just don't want things to get messier with weird build systems and multiple toolchains requirements though |
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pjp
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| Posted: Wed Jun 22, 2022 4:00 am Post subject: Re: |
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| GDH-gentoo wrote: | | pjp wrote: | Neither xbacklight nor xrandr have setuid/setgid bits set. [...]
What piece am I missing that allows either x11-apps/xbacklight or acpilight to make those changes? |
I don't know about acpilight, but xbacklight doesn't actually make the changes itself. It is an X client, and asks the X server to make them. |
That's what I was guessing. And as I'm using startx, I presumed that came into play, but I wasn't finding any details. And since running xbacklight as a normal user worked without special consideration, I had no clue how to even search for that information. I'm guessing only in code, which I'd almost certainly have trouble finding.
The acpilight version uses udev rules to change /sys/class/backlight/... group ownership to video, group writable. The code itself simply modifies those files. (a fancier 'echo $value > /path/to/file'). Since using echo worked after the original xbacklight stopped functioning, I wondered how other solutions worked, thinking they used some other "official" mechanism.
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