Unable to get brightness "current value"

[sefernet]

Like this :

$ ddcutil getvcp 0x10
VCP code 0x10 (Brightness                    ): current value =   255, max value =  100

The Brightness value its usable , when run "ddcutil setvcp 10 50" the screen is less bright, but the "getvcp" output remain "current value = 255" ... Without "current value", im thinking store some variable in a tmp file with a script as last resort...

What's going on ?

Any help would be apreciated ! Thanks !

---

$ ddcutil detect

Display 1
I2C bus: /dev/i2c-0
DRM_connector: card0-HDMI-A-1
EDID synopsis:
Mfg id: SAM - Samsung Electric Company
Model: LF27T35
Product code: 28799 (0x707f)
Serial number: H9VT506974
Binary serial number: 809906258 (0x30463052)
Manufacture year: 2022, Week: 18
VCP version: 2.0

---

$ ddcutil capabilities

Model: 550
MCCS version: 2.0
VCP Features:
Feature: 04 (Restore factory defaults)
Feature: 05 (Restore factory brightness/contrast defaults)
Feature: 08 (Restore color defaults)
Feature: 10 (Brightness)
Feature: 12 (Contrast)
Feature: 14 (Select color preset)
Values:
03: 4000 K
04: 5000 K
02: Display Native
07: 8200 K
08: 9300 K
0b: User 1
Feature: 16 (Video gain: Red)
Feature: 18 (Video gain: Green)
Feature: 1A (Video gain: Blue)
Feature: 60 (Input Source)
Values:
01: VGA-1
05: Composite video 1
06: Composite video 2
Feature: 87 (Sharpness)
Feature: B0 (Settings)
Values: 01 02 (interpretation unavailable)
Feature: B6 (Display technology type)
Feature: C6 (Application enable key)
Feature: C8 (Display controller type)
Feature: C9 (Display firmware level)
Feature: D6 (Power mode)
Values:
01: DPM: On, DPMS: Off
04: DPM: Off, DPMS: Off
Feature: DC (Display Mode)
Values:
01: Productivity
02: Mixed
03: Movie
04: User defined
05: Games
06: Sports
f0: Dynamic contrast
f1: Unrecognized value
f2: Unrecognized value
f9: Unrecognized value
fa: Unrecognized value
fb: Unrecognized value
Feature: DB (Image Mode)
Values:
00: No effect
04: Variable
fd: Unrecognized value
fe: Unrecognized value
Feature: DF (VCP Version)
Feature: EB (Manufacturer specific feature)
Values: 00 01 09 FD (interpretation unavailable)
Feature: EC (Manufacturer specific feature)
Values: 00 01 02 03 04 06 05 (interpretation unavailable)
Feature: F2 (Manufacturer specific feature)
Feature: F3 (Manufacturer specific feature)
Values: 00 01 02 (interpretation unavailable)
Feature: F6 (Manufacturer specific feature)
Feature: F7 (Manufacturer specific feature)
Values: 00 02 03 (interpretation unavailable)

[rockowitz]

It's a bug in the monitor's DDC implementation. I have a Samsung LS34A650U that behaves similarly. In my case, getvcp always returns 0 as the current value of feature x10. You probably want to use setvcp option --noverify to avoid the inevitable error message.

You could check if there's a firmware update for the monitor.

And yes, one way to work around this is to use scripts for getvcp and setvcp that maintain the current brightness value in an environment variable and/or a file.

[sefernet]

The culprit is a monitor setting that completely disables brightness adjustment in hardware.

Check a similar setting under "Menu > Picture > Eye saver Mode" (on my Samsung LF27T35) , must be "off" to enable brightness adjustment.

Now the output command correctly reports a value within the 0-100 range.

I hope its help

[rockowitz]

If I understand correctly what you're saying it's very strange. Initially, presumably with Eye Saver Mode on, you report that setvcp succeeds in changing the monitor brightness, but getvcp fails (always returning 0). With Eye Saver Mode off, setvcp still changes the monitor brightness, but getvcp returns a valid value. So Eye Saver Mode affects getvcp, but not setvcp.

I would expect that Eye Saver Mode would affect whether setvcp works, not whether getvcp works, This is normal, and is the case on my Samsung monitor that fails to correctly report the current brightness value, no matter the Eye Saver Mode setting.