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