[arch-general] Nvidia backlight control - acpi_video0/brightness changes - display doesn't?

[David C. Rankin]

On 11/19/2016 02:35 AM, Patrick Burroughs (Celti) via arch-general wrote:
> acpi_backlight=vendor is the default, acpi_backlight=none is invalid.
> The option you are looking for is acpi_backlight=video, which takes the
> backlight control away from the vendor driver and hands it back over to
> the standard ACPI backlight driver. This will generally solve the issue
> you are having.
> 
> ~Celti

Damn,

  And it worked so well that way :(  Thank you for identifying the
brightness/backlight issue. The Backlight wiki is where I first got the
acpi_backlight=none suggestion, see:

https://wiki.archlinux.org/index.php/Backlight#Kernel_command-line_options

  I've tried the acpi_backlight=video, and still no-joy (at least I still have
the acpi_video0 sysfs interface with that setting). The nice pop-up still
appears and shows the brightness adjustment, but the screen never changes.
(same symptoms as before)

  How is adjusting xrandr --brightness bad compared to adjusting
/sys/class/backlight/acpi_video0/brightness?? I understand setting xrandr
--brightness greater than 1.0 will draw additional power, etc.., but how is
setting it between 0.0 and 1.0 different than doing it with acpi? I'm just
trying to get it sorted out in my mind to understand the difference between
setting:

  /sys/class/backlight/acpi_video0/brightness

and

  xrandr --output LVDS-0 --brightness $foo

  I'll go search to see if I can find the difference, but if you have a
favorite link, I'd appreciate the help.

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

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