generate_override.py

import subprocess
import re
from pathlib import Path
from collections import namedtuple

Display = namedtuple('Display', ['vendor_id', 'product_id', 'name', 'edid'])


def get_ioreg_displays():
    """
    Query 'ioreg' for displays, and return the output as a string
    """
    cmd = ['ioreg', '-l', '-d0', '-w', '0', '-r', '-c', 'AppleDisplay']
    return subprocess.run(cmd, text=True, capture_output=True).stdout


def find_display_data(ioreg):
    """
    Finds (vendorid, productid, edid) tuples for displays in the ioreg data
    """
    edids = re.findall("IODisplayEDID.*?<([a-z0-9]+)>", ioreg)
    vendorids = re.findall("DisplayVendorID.*?([0-9]+)", ioreg)
    productids = re.findall("DisplayProductID.*?([0-9]+)", ioreg)
    return [Display(
        vendor_id = int(v),
        product_id = int(p),
        name = display_name_from_edid(e),
        edid = e
    ) for v, p, e in zip(vendorids, productids, edids)]


def display_name_from_edid(edid):
    """
    Extract the name of the display from the EDID data

    The monitor name is stored in the 13 bytes of text following "000000fc00".
    If the name is shorter, it is terminated with a newline (0a) and then
    padded with spaces.

    See https://en.wikipedia.org/wiki/Extended_Display_Identification_Data
    """
    display_name_descriptor = '000000fc00'
    if not display_name_descriptor in edid:
        return 'Display'

    from_index = edid.index(display_name_descriptor) + 10
    to_index = from_index + 26

    name_hex = edid[from_index:to_index]
    name = bytearray.fromhex(name_hex).decode().rstrip()

    return name


def path_for_override_file(display):
    """
    Determine the path where the override file should be written
    """
    vendorpath = "DisplayVendorID-%x" % display.vendor_id
    productpath = "DisplayProductID-%x" % display.product_id
    return Path('Overrides') / vendorpath / productpath


def generate_override_file(display, path):
    """
    Generates an override file for the display
    """
    if path.exists():
        return path

    print('Generating file', path)
    path.parent.mkdir(parents=True, exist_ok=True)

    path.touch()
    path.write_text(f"""\
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>DisplayProductName</key>
	<string>{display.name} (EDID override)</string>
	<key>edid-patches</key>
	<array>
		<dict>
			<key>offset</key>
			<integer>131</integer>
			<key>data</key>
			<data>wQ==</data>
		</dict>
	</array>
</dict>
</plist>""")


def print_command(display, path):
    """
    Print out the command to install the override
    """

    overrides = Path('/Library/Displays/Contents/Resources')
    source = path.absolute()
    target = overrides / path
    directory = target.parent

    if target.exists():
        print(f"Override is present for {display.name}. Remove it with the following command:")
        print(f"> sudo rm {target}")
    else:
        print(f"Override is not present for {display.name}. Install it with the following commands:")
        print(f"> sudo mkdir -p {directory}")
        print(f"> sudo cp {source} {target}")



def main():
    ioreg = get_ioreg_displays()
    displays = find_display_data(ioreg)
    for display in displays:
        print(f"Found display: {display.name}")
        path = path_for_override_file(display)
        generate_override_file(display, path)
        print_command(display, path)


if __name__ == "__main__":
    main()