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diff --git a/Documentation/EDID/HOWTO.txt b/Documentation/EDID/HOWTO.txt new file mode 100644 index 000000000..835db3322 --- /dev/null +++ b/Documentation/EDID/HOWTO.txt @@ -0,0 +1,58 @@ +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 (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. |