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, 15 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index d99fd9c0ec0..7f084b764fb 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -713,6 +713,21 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			The filter can be disabled or changed to another
 			driver later using sysfs.
 
+	drm_kms_helper.edid_firmware=[<connector>:]<file>
+			Broken monitors, graphic adapters and KVMs may
+			send no or incorrect EDID data sets. This parameter
+			allows to specify an EDID data set in the
+			/lib/firmware directory that is used instead.
+			Generic built-in EDID data sets are used, if one of
+			edid/1024x768.bin, edid/1280x1024.bin,
+			edid/1680x1050.bin, or edid/1920x1080.bin is given
+			and no file with the same name exists. Details and
+			instructions how to build your own EDID data are
+			available in Documentation/EDID/HOWTO.txt. An EDID
+			data set will only be used for a particular connector,
+			if its name and a colon are prepended to the EDID
+			name.
+
 	dscc4.setup=	[NET]
 
 	earlycon=	[KNL] Output early console device and options.