[Bug 65757] [snb pch eDP regression] Screen sometimes remains black after mode change - old fast & narrow link is more stable

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=65757

Egbert Eich <eich at pdx.freedesktop.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEEDINFO                    |ASSIGNED

--- Comment #7 from Egbert Eich <eich at pdx.freedesktop.org> ---
Thanks to Michal Srb, who took the task to perform this tedious investigation
we came to the following conclusions:

1. Of the eDP specific code paths we only need ironlake_edp_panel_on/off()
   and the call to intel_dp_init_panel_power_sequencer_registers() in
   intel_dp_init_connector() to make the panel work properly every time.

2. Some WARN() messages are triggered as the VDD enable/disable functions
   are not called. This however is harmless as the VDD doesn't seem to be
   controlled by the GPU as it seems (the panel works although the
   EDP_FORCE_VDD is off all the time).

3. Only enabling ironlake_edp_panel_off() will result in a dark screen
   all the time. This allows the conclusion that the panel power is controlled
   by the GPU.

If none of the ironlake_edp_panel_on/off() functions are called (as it is in
the DP case) the panel power remains on all the time (preset by the BIOS). 
For some reason the panel needs to undergo a power sequence during mode switch 
(ie when the DP lanes get retrained). Why this only needs to happen when two
lanes are active is not really clear - it may have to do with a brief loss of
the panel LVDS signaling which may case the panel to blank.

To me there are two conclusions possible:
a. the device should really be treated DEVICE_TYPE_eDP in the eDP BDB of VBT.
or
b. a fix is required for a device outside of the control of the driver (ie. the 
   Chrontel CH7511B (e)DP -> LVDS converter).

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