ddc-test.m

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 DISCLAIMER: I am not responsible of any damage caused by running this script, use it at your own risk!
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@import Darwin;
@import Foundation;
@import IOKit;

// clang -fmodules -o ddc-test ddc-test.m && ./ddc-test 50 0

typedef CFTypeRef IOAVServiceRef;
extern IOAVServiceRef IOAVServiceCreate(CFAllocatorRef allocator);

extern IOReturn IOAVServiceReadI2C(IOAVServiceRef service, uint32_t chipAddress, uint32_t offset, void* outputBuffer,
                                   uint32_t outputBufferSize);
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

int main(int argc, char** argv) {
    IOReturn err;
    IOAVServiceRef avService = IOAVServiceCreate(kCFAllocatorDefault);
    if (!avService) {
        NSLog(@"Can't open IOAVService: do you have an external monitor?");
        return 1;
    }
    
    char data[16];
    data[0] = 0x84;
    data[1] = 0x03;
    data[3] = 0x00;

    if (argc < 2 || argc > 3) {
      NSLog(@"Syntax %s <brightness 0-100> <volume 0-100>", argv[0]);
      return 1;
    }

    if (argc >= 2) {
      usleep(30000);
      data[2] = BRIGHTNESS;
      data[4] = atoi(argv[1]);
      data[5] = 0x6E ^ 0x51 ^ data[0] ^ data[1] ^ data[2] ^ data[3] ^ data[4];
      err = IOAVServiceWriteI2C(avService, 0x37, 0x51, data, 6);
      if (err) {
        NSLog(@"Something went wrong");
        return 1;
      }
    }
    if (argc == 3) {
      usleep(30000);
      data[2] = AUDIO_SPEAKER_VOLUME;
      data[4] = atoi(argv[2]);
      data[5] = 0x6E ^ 0x51 ^ data[0] ^ data[1] ^ data[2] ^ data[3] ^ data[4];
      err = IOAVServiceWriteI2C(avService, 0x37, 0x51, data, 6);
      if (err) {
        NSLog(@"Something went wrong");
        return 1;
      }
    }

    return 0;
}