[arch-general] ddcutil not working with nvidia card

[David C. Rankin]

On 1/23/2018 8:02 AM, Foxtrot Mike via arch-general wrote:
> I tried googling this error, but didn't find anything helpful.
> 
> So I tried using xbacklight instead. It didn't give any positive results either, even when I provided different kernel paramters as discribed in [1]. No matter what I do with acpi and kernel parameters,  "/sys/class/backlight/acpi_video0/brightness" doesn't give actual values. Also, 'xbacklight set' has no effect on screen brightness at all.
> 
> I also tried adding `Option "RegistryDwords" "EnableBrightnessControl=1"` in "/usr/share/X11/xorg.conf.d/10-nvidia-drm-outputclass.conf". Still keyboard brigthness keys couldn't change the brightness.
> 
> Can someone help me get ddcutil working with nvidia proprietary driver?

I doubt I can held get ddcutil working, but the problem is a lack of an
nvidia kernel module interface with /sys/class/backlight (which can be
frustrating). You do not want to use xbacklight -- which ironically does
not control actual backlight (display power), but instead controls pixel
darkness (the amount of black/white per-pixel). Reducing backlight does
not reduce display power, it just makes the pixels darker giving the
impression of reduced brightness. (this is really bad for laptop battery
life where reducing the display power is important)

You want xrandr. Query with `xrandr -q` and find your interface and you
can then set the desired brightness with something similar to

xrandr --output LVDS-0 --brightness 42

(where LVDS-0 is your monitor connections shown by --query)

Modern desktops generally integrate xrandr control (if not, you can
simply implement a short script that allows you to check and set the
desired brightness.

There were also several AUR packages (nvidia-bl, and nvidiabl or
similar) that provided a /sys/class/backlight kernel module that would
would work with most laptops, but I don't think they are maintained any
longer. (there was an example script on the X11 wiki some time ago for
doing this) That is another option.

-- 
David C. Rankin, J.D.,P.E.