Brightness change reverts instantly

[saeidhashemi]

Running the command line argument to reduce brightness on my external display works for about a second, then the brightness instantly reverts to what it was. Retrying the same command shows that the queried brightness has indeed gone back to its initial value, and the same thing repeats.

I have three external displays on a Macbook Pro 2019 running Catalina 10.15.6 (19G2021) and AMD graphics, driving them with Thunderbolt to DVI cables for two of them and Thunderbolt to HDMI for one of them. All of them were able to have their brightness adjusted with Windows PCs.

AMD Radeon Pro 5500M 4 GB
Intel UHD Graphics 630 1536 MB

I did a make for AMD.

% ./ddcctl -h         
D: NSScreen #188951239 (1920x1080 0°) 102.00 DPI
D: NSScreen #188951240 (1920x1080 0°) 102.00 DPI
D: NSScreen #188942017 (1920x1080 0°) 82.00 DPI
I: found 3 external displays

% ./ddcctl -d 1 -b 32-
D: NSScreen #188951239 (1920x1080 0°) 102.00 DPI
D: NSScreen #188951240 (1920x1080 0°) 102.00 DPI
D: NSScreen #188942017 (1920x1080 0°) 82.00 DPI
I: found 3 external displays
I: polling display 1's EDID
I: got edid.name: S22E200
I: got edid.serial: HCHJ404702
D: action: b: 32-
D: querying VCP control: #16 =?
I: VCP control #16 (0x10) = current: 64, max: 100
D: relative setting: 64 - 32 = 32
D: setting VCP control #16 => 32

Repeating this last line just shows me the exact same thing. And the setting has reverted to 64 each time.

[kfix]

Sounds like your Mac driver is doing something to assert DDC settings continuously, that the Windows one isn't?