Parse VGA configuration data (EDID) accessed from I2C device 0x50

edid.py

# Program to parse VGA data (EDID) accessed from I2C device 0x50
#
# This is a quick demo not a supported program, so don't expect
# correctness from it.
#
# Edid format from:
# https://en.wikipedia.org/wiki/Extended_Display_Identification_Data#EDID_1.4_data_format
#
# Ken Shirriff  http://righto.com

import re
import subprocess


def edid(bytes):
  assert(bytes[0] == 0)
  assert(bytes[1] == 0xff)
  assert(bytes[2] == 0xff)
  assert(bytes[3] == 0xff)
  assert(bytes[4] == 0xff)
  assert(bytes[5] == 0xff)
  assert(bytes[6] == 0xff)
  assert(bytes[7] == 0)

  print 'Header:'
  word89 = (bytes[8] << 8) | bytes[9]
  c0 = chr(64 + ((word89 >> 10) & 0x1f))
  c1 = chr(64 + ((word89 >> 5) & 0x1f))
  c2 = chr(64 + ((word89 >> 0) & 0x1f))
  print '  Manufacturer: %s%s%s' % (c0, c1, c2)

  print '  Product code: %d' % (bytes[10] | (bytes[11] << 8))
  print '  Serial number: %d' % (bytes[12] | (bytes[13] << 8) | (bytes[14] << 16) | (bytes[15] << 24))
  print '  Week: %d' % bytes[16]
  print '  Year: %d' % (bytes[16]+1990)
  print '  Edid version %d, revision %d' % (bytes[18], bytes[19])

  if bytes[20] & 128:
      print '  Digital input'
      print '  Depth: %s' % ['undef', '6', '8', '10', '12', '14', '16', 'res'][(bytes[20]>>4)&7]
      print '  Interface: %s' % ['undef', 'HDMIa', 'HDMIb', '?', 'MDDI', 'DisplayPort', '?', '?'][bytes[20]&7]
  else:
      print '  Analog input'
      print '  Levels: %s' % ['+0.7/-.03', '+.714/-.386', '+1.0/-.04', '+.7/0]'][(bytes[20]>>5)&3]
      if bytes[20] & 16:
        print '  Blank-to-black setup (pedestal) expected'
      if bytes[20] & 8:
        print '  Separate sync supported'
      if bytes[20] & 4:
        print '  Composite sync supported'
      if bytes[20] & 2:
        print '  Sync on green supported'
      if bytes[20] & 1:
        print '  VSync pulse must be serrated when composite or sync-on-green is used.'

  hsize = bytes[21]
  vsize = bytes[22]

  if hsize:
    print '  Horizontal screen size: %dcm' % hsize
  else:
    print '  Portrait aspect ratio: %.3f' % (vsize+99)/100.

  if vsize:
    print '  Vertical screen size: %dcm' % vsize
  else:
    print '  Landscapae aspect ratio: %.3f' % (hsize+99)/100.

  print '  Display gamma: %.3f' % (bytes[23]/100. + 1)

  if bytes[24] & 128: print '  DPMS standby supported'
  if bytes[24] & 64: print '  DPMS suspend supported'
  if bytes[24] & 32: print '  DPMS active-off supported'
  if bytes[20] & 128:
    print '  Display type (digital): %s' % ['RGB 4:4:4', 'RGB 4:4:4 + YCrCb 4:4:4', 'RGB 4:4:4 + YCrCb 4:2:2', 'RGB 4:4:4 + YCrCb 4:4:4 + YCrCb 4:2:2'][(bytes[24]>>3)&3]
  else:
    print '  Display type (analog): %s' % ['Monochrome or grayscale', 'RGB color', 'non-RGB color', 'undef'][(bytes[24]>>3)&3]
  if bytes[24] & 4: print '  Standard sRGB color space (data not printed)'
  if bytes[24] & 2: print '  Preferred timing mode in descriptor block 1'
  if bytes[24] & 1: print '  Continuous timings with GTF or CVT'

  rx = ((bytes[27] << 2) | ((bytes[25]>>6) & 3)) / 1024.
  ry = ((bytes[28] << 2) | ((bytes[25]>>4) & 3)) / 1024.
  gx = ((bytes[29] << 2) | ((bytes[25]>>2) & 3)) / 1024.
  gy = ((bytes[30] << 2) | ((bytes[25]>>0) & 3)) / 1024.
  bx = ((bytes[31] << 2) | ((bytes[26]>>6) & 3)) / 1024.
  by = ((bytes[32] << 2) | ((bytes[26]>>4) & 3)) / 1024.
  wx = ((bytes[33] << 2) | ((bytes[26]>>2) & 3)) / 1024.
  wy = ((bytes[34] << 2) | ((bytes[26]>>0) & 3)) / 1024.

  print 'Chromaticity coordinates:  r: (%.3f, %.3f), g: (%.3f, %.3f), b: (%.3f, %.3f), w: (%.3f, %.3f)' % (rx, ry, gx, gy, bx, by, wx, wy)

  timings = [
    '720x400 @ 70 Hz (VGA)',
    '720x400 @ 88 Hz (XGA)',
    '640x480 @ 60 Hz (VGA)',
    '640x480 @ 67 Hz (Apple Macintosh II)',
    '640x480 @ 72 Hz',
    '640x480 @ 75 Hz',
    '800x600 @ 56 Hz',
    '800x600 @ 60 Hz',
    '800x600 @ 72 Hz',
    '800x600 @ 75 Hz',
    '832x624 @ 75 Hz (Apple Macintosh II)',
    '1024x768 @ 87 Hz, interlaced (1024x768i)',
    '1024x768 @ 60 Hz',
    '1024x768 @ 72 Hz',
    '1024x768 @ 75 Hz',
    '1280x1024 @ 75 Hz',
    '1152x870 @ 75 Hz (Apple Macintosh II)',
    'manufacturer-specific 6',
    'manufacturer-specific 5',
    'manufacturer-specific 4',
    'manufacturer-specific 3',
    'manufacturer-specific 2',
    'manufacturer-specific 1',
    'manufacturer-specific 0',
  ]

  timingWord = (bytes[35] << 16) | (bytes[36] << 8) | bytes[37]
  print 'Established timings:'
  for i in range(0, 24):
    if timingWord & (1 << (23-i)):
      print '  %s' % timings[i]

  print 'Standard timing information:'
  for i in range(38, 54, 2):
    if bytes[i] != 1 or bytes[i+1] != 1:
      xres = (bytes[i]+31)*8
      aspect = [(16, 10), (4, 3), (5, 4), (16, 9)][bytes[i+1] >> 6]
      yres = xres * aspect[1] / aspect[0]
      vfreq = (bytes[i+1] & 63) + 60
      print '  X res: %d, aspect %d:%d, Y res (derived): %d), vertical frequency: %d' % (
        xres, aspect[0], aspect[1], yres, vfreq)

  for n in range(1, 5):
    i = n * 18 + 36
    if bytes[i] != 0 or bytes[i+1] != 0:
      detailedTimingDescriptor(n, bytes[i:i+18])
    else:
      otherMonitorDescriptors(n, bytes[i:i+18])
  if bytes[126] > 0:
    print '%d extensions (not displayed)' % bytes[126]

  if sum(bytes) % 256 != 0:
    print 'Bad checksum'

def detailedTimingDescriptor(n, bytes):
  print 'Descriptor %d: Detailed timing descriptor:' % n
  print '  Pixel clock: %dkHz' % ((bytes[0] | (bytes[1]<<8)) * 10)
  print '  Horizontal active pixels: %d' % (bytes[2] | ((bytes[4]&0xF0) << 4))
  print '  Horizontal blanking pixels: %d' % (bytes[3] | ((bytes[4]&0x0F) << 8))
  print '  Vertical active lines: %d' % (bytes[5] | ((bytes[7]&0xF0) << 4))
  print '  Vertical blanking lines: %d' % (bytes[6] | ((bytes[7]&0x0F) << 8))
  print '  Horizontal front porch pixels: %d' % (bytes[8] | ((bytes[11]&0xC0) << 2))
  print '  Horizontal sync pulse pixels: %d' % (bytes[9] | ((bytes[11]&0x30) << 4))
  print '  Vertical front porch lines: %d' % ((bytes[10]>>4) | ((bytes[11]&0x0C) << 2))
  print '  Vertical sync pulse lines: %d' % ((bytes[10]&0xF) | ((bytes[11]&0x03) << 4))
  print '  Horizontal image size: %dmm' % (bytes[12] | ((bytes[14]&0xF0) << 4))
  print '  Vertical image size: %dmm' % (bytes[13] | ((bytes[14]&0x0F) << 8))
  print '  Horizontal border pixels: %d' % bytes[15]
  print '  Vertical border lines: %d' % bytes[16]
  if bytes[17] & 0x80: print '  Interlaced'
  if bytes[17] & 0x60: print '  Stereo'
  if (bytes[17] & 0x10) == 0:
    print '  Analog'
    if bytes[17] & 4:
      print '  Bipolar analog composite'
    else:
      print '  Analog composite'
    if bytes[17] & 2: print '  VSync serration'
    if bytes[17] & 1:
      print '  Sync on all 3 RGB lines'
    else:
      print '  Sync on green only'
  elif (bytes[17] & 0x18) == 0x10:
    print '  Digital composite on HSync'
    if bytes[17] & 4:
      print '  Positive vertical sync polarity'
    else:
      print '  Negative vertical sync polarity'
  else:
    print '  Digital separate sync'
    if bytes[17] & 4: print '  VSync serration'
    if bytes[17] & 2:
      print '  Positive horizontal sync polarity'
    else:
      print '  Negative horizontal sync polarity'

def otherMonitorDescriptors(n, bytes):
  print 'Descriptor %d:' % n, 
  if bytes[3] == 0xFF: print 'Display serial number', text(bytes[5:18])
  elif bytes[3] == 0xFE: print 'Unspecified text', text(bytes[5:18])
  elif bytes[3] == 0xFD: displayRangeLimits(bytes[:18])
  elif bytes[3] == 0xFC: print 'Display name', text(bytes[5:18])
  elif bytes[3] == 0xFB: print 'Additional white point'
  elif bytes[3] == 0xFA: print 'Additional standard timing'
  elif bytes[3] == 0xF9: print 'Display color management'
  elif bytes[3] == 0xF8: print 'CVT timing codes'
  elif bytes[3] == 0xF7: print 'Additional standard timing'
  elif bytes[3] == 0x10: print 'Extended timing'
  else: print 'Unknown descriptor %x %s' % (bytes[3], text(bytes[5:18]))


def text(bytes):
  return re.sub('\n *', '', ''.join([chr(x) for x in bytes]))

def displayRangeLimits(bytes):
  print 'Display range limits'
  assert(bytes[0] == 0)
  assert(bytes[1] == 0)
  assert(bytes[2] == 0)
  assert(bytes[3] == 0xfd)
  print '  Minimum vertical field rate %dHz' % (bytes[5] + (256 if (bytes[4]&1) else 0))
  print '  Maximum vertical field rate %dHz' % (bytes[6] + (256 if (bytes[4]&2) else 0))
  print '  Minimum horizontal field rate %dHz' % (bytes[7] + (256 if (bytes[4]&4) else 0))
  print '  Maximum horizontal field rate %dHz' % (bytes[8] + (256 if (bytes[4]&8) else 0))
  print '  Maximum pixel clock rate: %dMhz' % (bytes[9] * 10)
  if bytes[10] == 0:
    print '  Default GTF'
  elif bytes[10] == 1:
    print '  No timing information'
  elif bytes[10] == 2:
    print '  Secondary GTF (uninterpreted)'
  elif bytes[10] == 4:
    print '  CVT (uninterpreted)'
  else:
    print '  Unexpected timing information byte %x' % bytes[10]

def main():
  # Get 128 bytes from the i2c output
  # Just parse i2cdump output instead of using an API
  i2c = subprocess.check_output("i2cdump -y 1 0x50 b", shell=True)
  bytes = []
  for line in i2c.split('\n')[1:9]:
    bytes.extend(int(x, 16) for x in line.split(' ')[1:17])
  edid(bytes)

if __name__ == '__main__':
  main()

There's small error in the decoder; I fixed it in
https://gist.github.com/przemekklosowski/93a82b9b171014b4d009a10ff35ce4d8
The output is used in your blog post on decoding VGA monitor EDID info, so for completeness maybe change it there too..
I am a fan of your posts---your debugging skills are epic, so I am quite honored to be able do debug the code of a debugging master, however tiny my fix is :)

This is sweet! Thanks!