Reading brightness/contrast doesn't work through USB-C to USB-C cable

[dahu33]

Hi,

I'm using a Dell U2720QM with a MacBook Pro (13-inch, 2018, Intel Iris Plus Graphics 655) on OS X 10.15.6 through a USB-C to USB-C cable (Gen 2).

While setting brightness/contrast works, reading/querying brightness/contrast doesn't work. My friend who own the same monitor had no problem reading brightness/contrast values with its MacBook Pro (15-inch, 2018, AMD Radeon Pro 560x) so I believe the issue is not on the monitor but rather on the Mac or software side.

I've already tried to play with various kDDCMinReplyDelay values without any success.

Here are the outputs.

$ ./ddcctl -d 1 -b \?
2020-08-20 19:40:31.301 ddcctl[4423:22754] D: NSScreen #722493133 (3008x1692 0°) HiDPI
2020-08-20 19:40:31.301 ddcctl[4423:22754] I: found 1 external display
2020-08-20 19:40:31.301 ddcctl[4423:22754] I: polling display 1's EDID

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:18947 depID:0 depIdx:0 Serial:

2020-08-20 19:40:31.314 ddcctl[4423:22754] I: got edid.serial: BB0MY13
2020-08-20 19:40:31.314 ddcctl[4423:22754] I: got edid.name: DELL U2720QM
2020-08-20 19:40:31.314 ddcctl[4423:22754] D: action: b: ?
2020-08-20 19:40:31.314 ddcctl[4423:22754] D: querying VCP control: #16 =?

D: IOI2CTransactionTypes: 0x13 (19)
D: IOI2CNoTransactionType                   supported 
D: IOI2CSimpleTransactionType               supported 
E: IOI2CDDCciReplyTransactionType           unsupported 
E: IOI2CCombinedTransactionType             unsupported 
D: IOI2CDisplayPortNativeTransactionType    supported 

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:15119 depID:0 depIdx:0 Serial:


D: IOI2CTransactionTypes: 0x13 (19)
D: IOI2CNoTransactionType                   supported 
D: IOI2CSimpleTransactionType               supported 
E: IOI2CDDCciReplyTransactionType           unsupported 
E: IOI2CCombinedTransactionType             unsupported 
D: IOI2CDisplayPortNativeTransactionType    supported 

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:18471 depID:0 depIdx:0 Serial:


D: IOI2CTransactionTypes: 0x13 (19)
D: IOI2CNoTransactionType                   supported 
D: IOI2CSimpleTransactionType               supported 
E: IOI2CDDCciReplyTransactionType           unsupported 
E: IOI2CCombinedTransactionType             unsupported 
D: IOI2CDisplayPortNativeTransactionType    supported 

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:15399 depID:0 depIdx:0 Serial:


D: IOI2CTransactionTypes: 0x13 (19)
D: IOI2CNoTransactionType                   supported 
D: IOI2CSimpleTransactionType               supported 
E: IOI2CDDCciReplyTransactionType           unsupported 
E: IOI2CCombinedTransactionType             unsupported 
D: IOI2CDisplayPortNativeTransactionType    supported 

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:15655 depID:0 depIdx:0 Serial:


D: IOI2CTransactionTypes: 0x13 (19)
D: IOI2CNoTransactionType                   supported 
D: IOI2CSimpleTransactionType               supported 
E: IOI2CDDCciReplyTransactionType           unsupported 
E: IOI2CCombinedTransactionType             unsupported 
D: IOI2CDisplayPortNativeTransactionType    supported 

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:18215 depID:0 depIdx:0 Serial:


D: IOI2CTransactionTypes: 0x13 (19)
D: IOI2CNoTransactionType                   supported 
D: IOI2CSimpleTransactionType               supported 
E: IOI2CDDCciReplyTransactionType           unsupported 
E: IOI2CCombinedTransactionType             unsupported 
D: IOI2CDisplayPortNativeTransactionType    supported 

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:15911 depID:0 depIdx:0 Serial:


D: IOI2CTransactionTypes: 0x13 (19)
D: IOI2CNoTransactionType                   supported 
D: IOI2CSimpleTransactionType               supported 
E: IOI2CDDCciReplyTransactionType           unsupported 
E: IOI2CCombinedTransactionType             unsupported 
D: IOI2CDisplayPortNativeTransactionType    supported 

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:16167 depID:0 depIdx:0 Serial:


D: IOI2CTransactionTypes: 0x13 (19)
D: IOI2CNoTransactionType                   supported 
D: IOI2CSimpleTransactionType               supported 
E: IOI2CDDCciReplyTransactionType           unsupported 
E: IOI2CCombinedTransactionType             unsupported 
D: IOI2CDisplayPortNativeTransactionType    supported 

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:16423 depID:0 depIdx:0 Serial:


D: IOI2CTransactionTypes: 0x13 (19)
D: IOI2CNoTransactionType                   supported 
D: IOI2CSimpleTransactionType               supported 
E: IOI2CDDCciReplyTransactionType           unsupported 
E: IOI2CCombinedTransactionType             unsupported 
D: IOI2CDisplayPortNativeTransactionType    supported 

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:17959 depID:0 depIdx:0 Serial:


D: IOI2CTransactionTypes: 0x13 (19)
D: IOI2CNoTransactionType                   supported 
D: IOI2CSimpleTransactionType               supported 
E: IOI2CDDCciReplyTransactionType           unsupported 
E: IOI2CCombinedTransactionType             unsupported 
D: IOI2CDisplayPortNativeTransactionType    supported 

Framebuffer: AppleIntelFramebuffer
IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU@2/AppleIntelFramebuffer@1/display0/AppleDisplay
VN:4268 PN:16830 SN:943075660 UN:1 IN:17703 depID:0 depIdx:0 Serial:

E: No data after 10 tries! 
2020-08-20 19:40:31.793 ddcctl[4423:22754] E: DDC send command failed!
2020-08-20 19:40:31.793 ddcctl[4423:22754] E: VCP control #16 (0x10) = current: 0, max: 0