| .. SPDX-License-Identifier: GPL-2.0 | |
| ==== | |
| EDID | |
| ==== | |
| 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. | |
| 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 |