docs: admin-guide: Move edid.rst from driver-api

This document describes actions that an admin can do, rather than
interfaces available to driver developers, so admin-guide seems to
be a more appropriate place for it.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

2 files changed, 61 insertions, 0 deletions

diff --git a/Documentation/admin-guide/edid.rst b/Documentation/admin-guide/edid.rst
new file mode 100644
index 000000000000..7dc07942ceb2
--- /dev/null
+++ b/Documentation/admin-guide/edid.rst
@@ -0,0 +1,60 @@
+.. 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
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index f1d0ccffbe72..5a6269fb8593 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -75,6 +75,7 @@ configure specific aspects of kernel behavior to your liking.
    cputopology
    dell_rbu
    device-mapper/index
+   edid
    efi-stub
    ext4
    nfs/index