linux/Documentation/EDID/HOWTO.txt
<<
>>
Prefs
   1In the good old days when graphics parameters were configured explicitly
   2in a file called xorg.conf, even broken hardware could be managed.
   3
   4Today, with the advent of Kernel Mode Setting, a graphics board is
   5either correctly working because all components follow the standards -
   6or the computer is unusable, because the screen remains dark after
   7booting or it displays the wrong area. Cases when this happens are:
   8- The graphics board does not recognize the monitor.
   9- The graphics board is unable to detect any EDID data.
  10- The graphics board incorrectly forwards EDID data to the driver.
  11- The monitor sends no or bogus EDID data.
  12- A KVM sends its own EDID data instead of querying the connected monitor.
  13Adding the kernel parameter "nomodeset" helps in most cases, but causes
  14restrictions later on.
  15
  16As a remedy for such situations, the kernel configuration item
  17CONFIG_DRM_LOAD_EDID_FIRMWARE was introduced. It allows to provide an
  18individually prepared or corrected EDID data set in the /lib/firmware
  19directory from where it is loaded via the firmware interface. The code
  20(see drivers/gpu/drm/drm_edid_load.c) contains built-in data sets for
  21commonly used screen resolutions (800x600, 1024x768, 1280x1024, 1600x1200,
  221680x1050, 1920x1080) as binary blobs, but the kernel source tree does
  23not contain code to create these data. In order to elucidate the origin
  24of the built-in binary EDID blobs and to facilitate the creation of
  25individual data for a specific misbehaving monitor, commented sources
  26and a Makefile environment are given here.
  27
  28To create binary EDID and C source code files from the existing data
  29material, simply type "make".
  30
  31If you want to create your own EDID file, copy the file 1024x768.S,
  32replace the settings with your own data and add a new target to the
  33Makefile. Please note that the EDID data structure expects the timing
  34values in a different way as compared to the standard X11 format.
  35
  36X11:
  37HTimings:  hdisp hsyncstart hsyncend htotal
  38VTimings:  vdisp vsyncstart vsyncend vtotal
  39
  40EDID:
  41#define XPIX hdisp
  42#define XBLANK htotal-hdisp
  43#define XOFFSET hsyncstart-hdisp
  44#define XPULSE hsyncend-hsyncstart
  45
  46#define YPIX vdisp
  47#define YBLANK vtotal-vdisp
  48#define YOFFSET (63+(vsyncstart-vdisp))
  49#define YPULSE (63+(vsyncend-vsyncstart))
  50
  51The CRC value in the last line
  52  #define CRC 0x55
  53also is a bit tricky. After a first version of the binary data set is
  54created, it must be checked with the "edid-decode" utility which will
  55most probably complain about a wrong CRC. Fortunately, the utility also
  56displays the correct CRC which must then be inserted into the source
  57file. After the make procedure is repeated, the EDID data set is ready
  58to be used.
  59