[Patch 0/3] Add ability to override a monitor's EDID information

Thorsten Schoel tschoel at web.de
Tue Apr 24 01:49:06 PDT 2012

Hello everybody,

having a monitor provide wrong or invalid EDID information – or none at
all – seems to be a problem quite a number of people are facing. A few
of the non-KMS X-drivers provide individual mechanisms for overriding
such an EDID with user provided data. To my knowledge at least the
kernel currently falls short of providing such a mechanism. This set of
patches attempts to provide one.

It has a number of potential shortcoming to which I was not yet able to
find any solution. These are:

1. It uses drm_get_connector_name to identify a connector. I am not
entirely sure though if these names are guaranteed to be unique (the
names used in /sys/class/drm seem to suggest they are not). Since they
are good enough for the kernel's video parameter, though, I figured they
should be good enough for the present purpose as well.

2. One of the patches introduces a new kernel parameter (edid_override).
This parameter takes either the raw edid-data encoded as a hex-string or
the name of a firmware file from which to load the data. I am a bit
unhappy with both solutions since the first puts raw data on the command
line and makes it rather long and the second… well edid-data is not
exactly firmware in the strict sense. It may be firm, but it is probably
not ware.

3. I thought about making the sysfs code more sophisticated so that
individual bytes could be written at any position within the data. Yet,
there seems to be no way to truncate a file in sysfs which would be
important when changing the number of 128 byte blocks the data is made
up of. In addition, I cannot see what good such a behaviour would be or
how the obscure use probably is somewhere out there could justify the
additional complexity. I therefore opted for the approach of only
allowing writes of complete edid-blobs in one pass.

The patches I submit were tested on an x64 with radeon graphics and on
an x86 with i915. They were made for 3.4.

 drivers/gpu/drm/drm_edid.c  |  399 +++++++++++++++++++++++++++++++++++++++++++-
 drivers/gpu/drm/drm_sysfs.c |   43 ++++
 include/drm/drm_crtc.h      |    2 
 3 files changed, 435 insertions(+), 9 deletions(-)

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