drm: allow loading an EDID as firmware to override broken monitor

Broken monitors and/or broken graphic boards may send erroneous or no
EDID data. This also applies to broken KVM devices that are unable to
correctly forward the EDID data of the connected monitor but invent
their own fantasy data.

This patch allows to specify an EDID data set to be used instead of
probing the monitor for it. It contains built-in data sets of frequently
used screen resolutions. In addition, a particular EDID data set may be
provided in the /lib/firmware directory and loaded via the firmware
interface. The name is passed to the kernel as module parameter of the
drm_kms_helper module either when loaded
  options drm_kms_helper edid_firmware=edid/1280x1024.bin
or as kernel commandline parameter
  drm_kms_helper.edid_firmware=edid/1280x1024.bin

It is also possible to restrict the usage of a specified EDID data set
to a particular connector. This is done by prepending the name of the
connector to the name of the EDID data set using the syntax
  edid_firmware=[<connector>:]<edid>
such as, for example,
  edid_firmware=DVI-I-1:edid/1920x1080.bin
in which case no other connector will be affected.

The built-in data sets are
Resolution    Name
--------------------------------
1024x768      edid/1024x768.bin
1280x1024     edid/1280x1024.bin
1680x1050     edid/1680x1050.bin
1920x1080     edid/1920x1080.bin

They are ignored, if a file with the same name is available in the
/lib/firmware directory.

The built-in EDID data sets are based on standard timings that may not
apply to a particular monitor and even crash it. Ideally, EDID data of
the connected monitor should be used. They may be obtained through the
drm/cardX/cardX-<connector>/edid entry in the /sys/devices PCI directory
of a correctly working graphics adapter.

It is even possible to specify the name of an EDID data set on-the-fly
via the /sys/module interface, e.g.
echo edid/myedid.bin >/sys/module/drm_kms_helper/parameters/edid_firmware
The new screen mode is considered when the related kernel function is
called for the first time after the change. Such calls are made when the
X server is started or when the display settings dialog is opened in an
already running X server.

Signed-off-by: Carsten Emde <C.Emde@osadl.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 39 insertions, 0 deletions

diff --git a/Documentation/EDID/HOWTO.txt b/Documentation/EDID/HOWTO.txt
new file mode 100644
index 000000000000..75a9f2a0c43d
--- /dev/null
+++ b/Documentation/EDID/HOWTO.txt
@@ -0,0 +1,39 @@
+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 (1024x768, 1280x1024, 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 and
+replace the settings with your own data. The CRC value in the last line
+  #define CRC 0x55
+is a bit tricky. After a first version of the binary data set is
+created, it must be 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.