i2cwrite.m

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

// clang -fmodules -o i2cwrite i2cwrite.m && ./i2cwrite

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 root?");
    return 1;
  }

  IOReturn err;
  UInt8 data[256];
  memset(data, 0, sizeof(data));

  // MARK: Write BRIGHTNESS
  struct DDCWriteCommand command;
  command.control_id = BRIGHTNESS;
  command.new_value = 70;
  NSLog(@"\nSetting BRIGHTNESS to %d", command.new_value);

  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];

  for (int i = 0; i < 3; ++i)
  {
    err = IOAVServiceWriteI2C(avService, 0x37, 0x51, data, 6);
    if (err) {
        NSLog(@"i2c error: %s", mach_error_string(err));
    }
    usleep(50000);
  }

  NSData *nsdata = [NSData dataWithBytes:(const void *)data length:(NSUInteger)6];
  NSLog(@"Written to i2c: %@", nsdata);


  // MARK: Read BRIGHTNESS

  data[0] = 0x82;
  data[1] = 0x01;
  data[2] = BRIGHTNESS;
  data[3] = 0x6e ^ data[0] ^ data[1] ^ data[2] ^ data[3];

  char i2cBytes[12];
  memset(i2cBytes, 0, sizeof(i2cBytes));

  NSData *sendingData = [NSData dataWithBytes:(const void *)data length:(NSUInteger)4];
  NSLog(@"sending: %@", sendingData);

  usleep(50000);
  err = IOAVServiceWriteI2C(avService, 0x37, 0x51, data, 4);
  if (err) {
      NSLog(@"i2c error: %s", mach_error_string(err));
      return 1;
  }

  usleep(50000);
  err = IOAVServiceReadI2C(avService, 0x37, 0x51, i2cBytes, 12);

  if (err) {
    NSLog(@"i2c error: %s", mach_error_string(err));
  } else {
    NSData *readData = [NSData dataWithBytes:(const void *)i2cBytes length:(NSUInteger)11];
    NSLog(@"readbuf: %@", readData);

    NSRange maxValueRange = {7, 1};
    NSRange currentValueRange = {9, 1};
    char maxValue, currentValue;
    [[readData subdataWithRange:maxValueRange] getBytes:&maxValue length:sizeof(1)];
    [[readData subdataWithRange:currentValueRange] getBytes:&currentValue length:sizeof(1)];
    NSLog(@"\nMAX: %d\nCURRENT: %d", maxValue, currentValue);
  }

  return 0;
}