|In the good old days when graphics parameters were configured explicitly
|in a file called xorg.conf, even broken hardware could be managed.
|Today, with the advent of Kernel Mode Setting, a graphics board is
|either correctly working because all components follow the standards -
|or the computer is unusable, because the screen remains dark after
|booting or it displays the wrong area. Cases when this happens are:
|- The graphics board does not recognize the monitor.
|- The graphics board is unable to detect any EDID data.
|- The graphics board incorrectly forwards EDID data to the driver.
|- The monitor sends no or bogus EDID data.
|- A KVM sends its own EDID data instead of querying the connected monitor.
|Adding the kernel parameter "nomodeset" helps in most cases, but causes
|restrictions later on.
|As a remedy for such situations, the kernel configuration item
|CONFIG_DRM_LOAD_EDID_FIRMWARE was introduced. It allows to provide an
|individually prepared or corrected EDID data set in the /lib/firmware
|directory from where it is loaded via the firmware interface. The code
|(see drivers/gpu/drm/drm_edid_load.c) contains built-in data sets for
|commonly used screen resolutions (800x600, 1024x768, 1280x1024, 1600x1200,
|1680x1050, 1920x1080) as binary blobs, but the kernel source tree does
|not contain code to create these data. In order to elucidate the origin
|of the built-in binary EDID blobs and to facilitate the creation of
|individual data for a specific misbehaving monitor, commented sources
|and a Makefile environment are given here.
|To create binary EDID and C source code files from the existing data
|material, simply type "make".
|If you want to create your own EDID file, copy the file 1024x768.S,
|replace the settings with your own data and add a new target to the
|Makefile. Please note that the EDID data structure expects the timing
|values in a different way as compared to the standard X11 format.
|HTimings: hdisp hsyncstart hsyncend htotal
|VTimings: vdisp vsyncstart vsyncend vtotal
|#define XPIX hdisp
|#define XBLANK htotal-hdisp
|#define XOFFSET hsyncstart-hdisp
|#define XPULSE hsyncend-hsyncstart
|#define YPIX vdisp
|#define YBLANK vtotal-vdisp
|#define YOFFSET (63+(vsyncstart-vdisp))
|#define YPULSE (63+(vsyncend-vsyncstart))
|The CRC value in the last line
| #define CRC 0x55
|also is a bit tricky. After a first version of the binary data set is
|created, it must be checked with the "edid-decode" utility which will
|most probably complain about a wrong CRC. Fortunately, the utility also
|displays the correct CRC which must then be inserted into the source
|file. After the make procedure is repeated, the EDID data set is ready
|to be used.