[Bug 85924] [hsw DP-to-DVI] DP hot plug fails if booted without Monitor Attached

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=85924

--- Comment #46 from Todd Previte <tprevite at gmail.com> ---
Created attachment 115826
  --> https://bugs.freedesktop.org/attachment.cgi?id=115826&action=edit
Add loop to drm_get_edid() to make multiple attempts to probe the DDC

Here is a potential fix for this problem. Please test with the attached patch
and report back here. 

Details:

drm_get_edid() only makes one attempt to probe the DDC before returning NULL.
Some passive DP dongles will fail to operate correctly in this case because
they need that initial transaction to "wake up" and get things running. Each
time an I2C transaction needs to be done, this probe must occur or the device
will not respond. In the case of the dongles, this has to happen before the
EDID can be read out of the device. 

This patch corrects this problem by probing the DDC up to 5 times before giving
up and returning NULL, waiting for 10-11ms between attempts to give the device
time to get setup. Once the DDC probe completes successfully, the EDID can then
be read normally from the device.

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