Re: GNOME brightness controls have no effect on brightness

[davidson <davidson@freevolt.org>]

On Mon, 4 Apr 2022 Juan R.D. Silva wrote:
> On 2022-04-04 5:17 a.m., davidson wrote:
> > On Sun, 3 Apr 2022 Juan R.D. Silva wrote:
> > > Hi folks,
> > >
> > > Debian 11 on Dell M4800 Mobile Workstation with Nvidia Quadro K2100M card.
> > >
> > > I've been using the system with full (100%) screen brightness for rather 
> > > long time. Today I wanted to reduce it and to my astonishment found that 
> > > GNOME "progressed" to the point when even this basic feature has gone.
> > >
> > > The function keys work, the sliders move. Everything is dandy, except it 
> > > has no effect on the actual brightness, which stack at 100%. Even setting 
> > > the brightness in BIOS to 50% instead of 100% has no effect.
> > >
> > >
> > > Google revealed a lot of complaints but so far I've not found any really 
> > > helpful suggestion.  Worse, I even couldn't find any trace of this being a 
> > > bug of some sort.
> > >
> > > Anyone can help here? I'm getting old and my eyes sore. :-(
> >
> > I'll show you what I do. I don't use GNOME, but maybe it will work.
> >
> > Four steps:
> >
> > 1. Confirm this directory has expected contents:
> >
> >    $ ls -p /sys/class/backlight/acpi_video0/
> >    actual_brightness  bl_power  brightness  device  max_brightness  power/  
> > subsystem  type  uevent
> >
> > It has "brightness" and "max_brightness" files. So far so good.
> >
> >
> > 2. Find out what counts as maximum brightness. (I think I have seen
> > this vary from laptop to laptop):
> >
> >   $ cat /sys/class/backlight/acpi_video0/max_brightness
> >   7
> >
> >
> > 3. Find out what value current brightness is:
> >
> >   $ cat /sys/class/backlight/acpi_video0/brightness
> >   5
> >
> >
> > 4. As root, write some value less-than-or-equal-to maximum brightness
> > (found in step 2) into "brightness" file. For example, when 5 is too
> > bright, I might try replacing that with 4.
> >
> >   # echo -n 4 > /sys/class/backlight/acpi_video0/brightness
> >
> > That is all.
> >
> >
> > Hope this helps.
>
> > > Sorry for the rant but it looks that either GNOME folks decided they
> > > know it best what it should be for everyone or they are so busy with
> > > frequent "nice" GUI changes and re-designs that there is no time
> > > left for actual functionality. Fake controls and sliders doing
> > > nothing?.. This is really climax.
> >
> > I have no patience for figuring out where graphical desktops hide (or
> > document) their "user friendly" configuration tools, so I can
> > sympathise.
> >
> > Something like the above procedure (maybe with some extra exploration
> > of /sys/class) has always worked for me.
> >
> > Good luck. Hope this helps.
>
> Looks that I failed to deliver the point in my post.
>
> All 4 steps are correct. And I most certainly can happily change the value in 
> the /sys/class/backlight/acpi_video0/brightness file to whatever I want. The 
> problem is it does not have any effect on the actual/physical screen 
> brightness.

LOL. Sorry to hear this.

I hope somebody who knows how this stuff works will pick up the
thread and enlighten us.

In the mean time, out of curiosity I searched for terms

 /sys/class/backlight/acpi_video0/brightness ineffective

and got some results that were informative to me (in my infinite
ignorance).

The question in this one was interesting,

 https://askubuntu.com/questions/1045624/how-to-switch-between-options-in-sys-class-backlight-to-solve-brightness-prob

...because it looks to me that the questioner has the same symptoms as
you, but *can* manipulate screen brightness using

  /sys/class/backlight/nv_backlight/*

files instead.

This made me wonder if you have a directory

 /sys/class/backlight/nv_backlight/

If you do, you could try the same four ineffective steps as before,
except replace "acpi_video0" with "nv_backlight".


--
Ce qui est important est rarement urgent
et ce qui est urgent est rarement important
-- Dwight David Eisenhower