EDID¶

In the good old days when graphics parameters were configured explicitlyin a file called xorg.conf, even broken hardware could be managed.

Today, with the advent of Kernel Mode Setting, a graphics board iseither correctly working because all components follow the standards -or the computer is unusable, because the screen remains dark afterbooting 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 causesrestrictions later on.

As a remedy for such situations, the kernel configuration itemCONFIG_DRM_LOAD_EDID_FIRMWARE was introduced. It allows to provide anindividually prepared or corrected EDID data set in the /lib/firmwaredirectory from where it is loaded via the firmware interface. The code(see drivers/gpu/drm/drm_edid_load.c) contains built-in data sets forcommonly used screen resolutions (800x600, 1024x768, 1280x1024, 1600x1200,1680x1050, 1920x1080) as binary blobs, but the kernel source tree doesnot contain code to create these data. In order to elucidate the originof the built-in binary EDID blobs and to facilitate the creation ofindividual data for a specific misbehaving monitor, commented sourcesand a Makefile environment are given here.

To create binary EDID and C source code files from the existing datamaterial, simply type “make” in tools/edid/.

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 theMakefile. Please note that the EDID data structure expects the timingvalues in a different way as compared to the standard X11 format.

X11:

HTimings:

hdisp hsyncstart hsyncend htotal

VTimings:

vdisp vsyncstart vsyncend vtotal

EDID:

#define XPIX hdisp#define XBLANK htotal-hdisp#define XOFFSET hsyncstart-hdisp#define XPULSE hsyncend-hsyncstart#define YPIX vdisp#define YBLANK vtotal-vdisp#define YOFFSET vsyncstart-vdisp#define YPULSE vsyncend-vsyncstart