M1 External Monitor Brightness Control over DDC

brt.m

@import Darwin;
@import Foundation;
@import IOKit;

// clang -fmodules -o brt brt.m && ./brt

// credit to @zhuowei for discovering the following APIs
typedef CFTypeRef IOAVServiceRef;
extern IOAVServiceRef IOAVServiceCreate(CFAllocatorRef allocator);
extern IOReturn IOAVServiceCopyEDID(IOAVServiceRef service, CFDataRef* x2);
// outputBufferSize must be less than (1 << 12) (4096 bytes)
extern IOReturn IOAVServiceReadI2C(IOAVServiceRef service, uint32_t chipAddress, uint32_t offset, void* outputBuffer,
                                   uint32_t outputBufferSize);
// This didn't work for me (returned no error, but EDID not written)
extern IOReturn IOAVServiceWriteI2C(IOAVServiceRef service, uint32_t chipAddress, uint32_t dataAddress, void* inputBuffer,
                                    uint32_t inputBufferSize);

#define BRIGHTNESS 0x10
#define CONTRAST 0x12
#define AUDIO_SPEAKER_VOLUME 0x62
#define AUDIO_MUTE 0x8D

struct DDCWriteCommand
{
    UInt8 control_id;
    UInt8 new_value;
};


int main(int argc, char** argv) {
    
    IOAVServiceRef avService = IOAVServiceCreate(kCFAllocatorDefault);
    if (!avService) {
        NSLog(@"Can't open IOAVService: do you have an external monitor? Are you on M1?");
        return 1;
    }
    
    char data[16];
    struct DDCWriteCommand command;
    command.control_id = BRIGHTNESS;
    if (argc == 2) {
        command.new_value = atoi(argv[1]);
    }
    else {
        command.new_value = 1;
    }
    

    data[0] = 0x84;
    data[1] = 0x03;
    data[2] = command.control_id;
    data[3] = (command.new_value) >> 8;
    data[4] = command.new_value & 255;
    data[5] = 0x6E ^ 0x51 ^ data[0] ^ data[1] ^ data[2] ^ data[3] ^ data[4];
    IOReturn err = IOAVServiceWriteI2C(avService, 0x37, 0x51, data, 6);
    return err;
}