| .. 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 |
