This should change the brightness level of a single external display connected to an M1 MBP or MBA relative to the current setting using DDC/CI. Reportedly does not work on M1 Mini.

chbrt.m

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

/*******

This should change the brightness level of a single external display connected to an M1 MBP or MBA relative to the current setting using DDC/CI. Reportedly does not work on M1 Mini.
Credits to @tao-j and @alin23

Compile:

clang -fmodules -o chbrt chbrt.m

To increase brightness by 10% enter:

./chbrt 10

To decrease brightness by 10% enter:

./chbrt -10

*******/

typedef CFTypeRef IOAVServiceRef;
extern IOAVServiceRef IOAVServiceCreate(CFAllocatorRef allocator);
extern IOReturn IOAVServiceCopyEDID(IOAVServiceRef service, CFDataRef* x2);
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
#define SLEEPTIME 5000

struct DDCWriteCommand {
	UInt8 control_id;
	UInt8 new_value;
};

int main(int argc, char** argv) {
	IOAVServiceRef avService = IOAVServiceCreate(kCFAllocatorDefault);
	if (!avService) {
		NSLog(@"This is a tool to change the brightness of a single external display connected to an M1 Mac via DDC/CI. Do you have an external monitor? Are you on M1? Are you root?");
		return 1;
	}

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

	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];

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

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

	NSRange maxValueRange = {7, 1};
	NSRange currentValueRange = {9, 1};

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

		char maxValue, currentValue;
		[[readData subdataWithRange:maxValueRange] getBytes:&maxValue length:sizeof(1)];
		[[readData subdataWithRange:currentValueRange] getBytes:&currentValue length:sizeof(1)];

		NSLog(@"Current brightness is %i", currentValue);

		if (argc == 2) {  

			int nextValue;
			nextValue = currentValue+atoi(argv[1]);

			if (nextValue<0) nextValue=0;
			if (nextValue>maxValue) nextValue=maxValue;

			struct DDCWriteCommand command; 
			command.control_id = BRIGHTNESS;

			NSLog(@"Setting brightness to %i", nextValue);

			command.new_value = nextValue;

			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(SLEEPTIME);
			}

			NSData *nsdata = [NSData dataWithBytes:(const void *)data length:(NSUInteger)6];
		} else {
			NSLog(@"Please specify a value between -100 and 100 as an argument to increase/decrease brightness by the desired level.");
		}
	}
	return 0;
}

I've found the solution:
https://gist.github.com/ybbond/f3bdcd8b7faa21b6e348c2126fc4eb07

I've found the solution:
https://gist.github.com/ybbond/f3bdcd8b7faa21b6e348c2126fc4eb07

Amazing! Sorry for not responding earlier.

If you want, you can use Hammerspoon to bind the script to the real brightness up/down media keys easily.

I also have a little swift terminal app to show the system OSD (for brightness, volume levels, etc) instead of notifications. You can incorporate it into a Hammerspoon script so you can basically replicate MonitorControl (until it does not have full M1 support like Lunar). Here is the link for the app (you have to build it):

https://github.com/waydabber/showosd

Here is an example Hammerspoon script does something similar (it's not for brightness control but to control my Yamaha AV receiver via the volume keys and showing the system OSD). This can be easily remade to be used for brightness control as well:

https://gist.github.com/waydabber/3241fc146cef65131a42ce30e4b6eab7

If you want, you can use Hammerspoon to bind the script to the real brightness up/down media keys easily.

hi, thanks for the input, I currently use Keyboard Maestro (KM). I tried Hammerspoon before (and created some spoon), but KM does the thing I do in Hammer before... and more.

https://github.com/waydabber/showosd

but this repo makes me want to try Hammerspoon again... thanks! Yea, the beautiful thing about macOS is the diversity of tool to automate the OS. It's like the proprietary version of Arch Linux (controversial statement, but it's just how I perceive it 😋)

Here is a much more improved version to control the display:

https://github.com/waydabber/m1ddc