Two identical Samsung LU28R55 connected. Only one can be controlled.

[ndemarco]

I'm seeing both Samsung LU28R55 monitors in probe.log, but only one presents a control section. Both are connected through a cable (samsung provided) directly to a Plugable thunderbolt 3 breakout hub.

I spent some time searching for similar issues, but no luck. Any pointers? Is this a known situation?

[Date: 5/24/2024 10:06:59 AM Ver: 4.6.14.0]
{
  "System": "Manufacturer: Dell Inc., Model: XPS 9320, OS: 10.0.22631.0",
  "Device Context - DeviceItems": [
    {
      "DeviceInstanceId": "DISPLAY\\SHP1551\\4&255e004d&0&UID8388688",
      "Description": "Dell XPS 9320 SHP1551 Display",
      "DisplayIndex": 1,
      "MonitorIndex": 0
    },
    {
      "DeviceInstanceId": "DISPLAY\\SAM1015\\4&255e004d&0&UID4165",
      "Description": "Generic PnP Monitor",
      "DisplayIndex": 2,
      "MonitorIndex": 0
    },
    {
      "DeviceInstanceId": "DISPLAY\\SAM1015\\4&255e004d&0&UID20550",
      "Description": "Generic PnP Monitor",
      "DisplayIndex": 3,
      "MonitorIndex": 0
    }
  ],
  "DisplayMonitor - DisplayItems": [
    {
      "DeviceInstanceId": "DISPLAY\\SAM1015\\4&255e004d&0&UID20550",
      "DisplayName": "LU28R55",
      "NativeResolution": {
        "_height": 2160,
        "_width": 3840
      },
      "PhysicalSize": {
        "_height": 14.173228263854981,
        "_width": 24.881889343261719
      },
      "PhysicalDiagonalLength": 28.6354465,
      "IsInternal": false,
      "ConnectionDescription": "DisplayPort"
    },
    {
      "DeviceInstanceId": "DISPLAY\\SHP1551\\4&255e004d&0&UID8388688",
      "DisplayName": "",
      "NativeResolution": {
        "_height": 2400,
        "_width": 3840
      },
      "PhysicalSize": {
        "_height": 7.08661413192749,
        "_width": 11.338582992553711
      },
      "PhysicalDiagonalLength": 13.3709974,
      "IsInternal": true,
      "ConnectionDescription": "Internal"
    },
    {
      "DeviceInstanceId": "DISPLAY\\SAM1015\\4&255e004d&0&UID4165",
      "DisplayName": "LU28R55",
      "NativeResolution": {
        "_height": 2160,
        "_width": 3840
      },
      "PhysicalSize": {
        "_height": 14.173228263854981,
        "_width": 24.881889343261719
      },
      "PhysicalDiagonalLength": 28.6354465,
      "IsInternal": false,
      "ConnectionDescription": "DisplayPort"
    }
  ],
  "Display Config - DisplayItems": [
    {
      "DeviceInstanceId": "DISPLAY\\SHP1551\\4&255e004d&0&UID8388688",
      "DisplayName": "",
      "IsInternal": true,
      "RefreshRate": 59.9939232,
      "ConnectionDescription": "Internal",
      "IsAvailable": true
    },
    {
      "DeviceInstanceId": "DISPLAY\\SAM1015\\4&255e004d&0&UID4165",
      "DisplayName": "LU28R55",
      "IsInternal": false,
      "RefreshRate": 59.996624,
      "ConnectionDescription": "DisplayPort",
      "IsAvailable": true
    },
    {
      "DeviceInstanceId": "DISPLAY\\SAM1015\\4&255e004d&0&UID20550",
      "DisplayName": "LU28R55",
      "IsInternal": false,
      "RefreshRate": 59.996624,
      "ConnectionDescription": "DisplayPort",
      "IsAvailable": true
    }
  ],
  "Device Installation - InstalledItems": [
    {
      "DeviceInstanceId": "DISPLAY\\SAM1015\\4&255E004D&0&UID20550",
      "Description": "Generic PnP Monitor",
      "IsRemovable": true
    },
    {
      "DeviceInstanceId": "DISPLAY\\SHP1551\\4&255E004D&0&UID8388688",
      "Description": "Dell XPS 9320 SHP1551 Display",
      "IsRemovable": false
    },
    {
      "DeviceInstanceId": "DISPLAY\\SAM1015\\4&255E004D&0&UID4165",
      "Description": "Generic PnP Monitor",
      "IsRemovable": true
    }
  ],
  "Monitor Configuration - PhysicalItems": [
    {
      "Key": {
        "DisplayIndex": 1,
        "MonitorRect": "Location:0,0, Size:3840,2400"
      },
      "Value": [
        {
          "Description": "Dell XPS 9320 SHP1551 Display",
          "MonitorIndex": 0,
          "Capability": {
            "IsHighLevelBrightnessSupported": false,
            "IsLowLevelBrightnessSupported": false,
            "IsContrastSupported": false,
            "IsPrecleared": false,
            "CapabilitiesString": null,
            "CapabilitiesReport": null,
            "CapabilitiesData": null
          },
          "GetBrightness": "Success: False",
          "SetBrightness": "Success: False"
        }
      ]
    },
    {
      "Key": {
        "DisplayIndex": 2,
        "MonitorRect": "Location:-1811,-2160, Size:3840,2160"
      },
      "Value": [
        {
          "Description": "Generic PnP Monitor",
          "MonitorIndex": 0,
          "Capability": {
            "IsHighLevelBrightnessSupported": false,
            "IsLowLevelBrightnessSupported": false,
            "IsContrastSupported": false,
            "IsPrecleared": false,
            "CapabilitiesString": null,
            "CapabilitiesReport": null,
            "CapabilitiesData": null
          },
          "GetBrightness": "Success: True, Valid: True (Minimum: 0, Current: 100, Maximum: 100)",
          "SetBrightness": "Success: True, Match: True (Expected: 93, Actual: 93)"
        }
      ]
    },
    {
      "Key": {
        "DisplayIndex": 3,
        "MonitorRect": "Location:2029,-2160, Size:3840,2160"
      },
      "Value": [
        {
          "Description": "Generic PnP Monitor",
          "MonitorIndex": 0,
          "Capability": {
            "IsHighLevelBrightnessSupported": false,
            "IsLowLevelBrightnessSupported": true,
            "IsContrastSupported": true,
            "IsPrecleared": false,
            "CapabilitiesString": "(prot(monitor)type(lcd)model(FALCON)cmds(01 02 03 07 0C 4E F3 E3)vcp(02 04 05 08 0B 0C 10 12 14(01 04 05 06 07 08 0A 0B) 16 18 1A 6C 6E 70 AC AE B6 C0 C6 C8 C9 CA CC(00 02 03 04 05 08 09 0A 0D) D6(01 04) DC(00 01 02 03 04) DF 60(01 03) 62 8D FF)mswhql(1)mccs_ver(2.0)asset_eep(32)mpu_ver(01))",
            "CapabilitiesReport": "Luminance: True, Contrast: True, Temperature: True, Input Source: True, Speaker Volume: True, Power Mode: True",
            "CapabilitiesData": null
          },
          "GetBrightness": "Success: True, Valid: True (Minimum: 0, Current: 100, Maximum: 100)",
          "SetBrightness": "Success: True, Match: True (Expected: 94, Actual: 94)"
        }
      ]
    }
  ],
  "MSMonitorClass - DesktopItems": [
    {
      "DeviceInstanceId": "DISPLAY\\SHP1551\\4&255E004D&0&UID8388688",
      "Description": "Dell XPS 9320 SHP1551 Display",
      "BrightnessLevels": "0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100"
    }
  ],
  "ElapsedTime": [
    "DeviceItems    -> 0.016",
    "DisplayMonitorItems -> 0.017",
    "DisplayConfigItems -> 0.016",
    "InstalledItems -> 0.030",
    "PhysicalItems  -> 4.760",
    "DesktopItems   -> 0.049"
  ]
}

[emoacht]

As you see, one of Sumsung monitors does not report capabilities information.
I have no idea why but it seems to respond to get and set brightness commands. Thus you can use /preclear option. See #270 (comment)