Here's a fun sequence:
$ xrandr --output LVDS1 --auto --crtc 1 $ sudo chvt 1
(then switch back to X)
The LVDS goes black. Why? The LVDS is *disabled*. Turns out that the cause was that after the mode was all nicely set, intel_disable_pch_ports went and turned it back off.
Well, a couple of weeks ago, we found that intel_disable_pch_ports was doing the same thing to DP ports, and the cause was that the way to tell which port a DP output was running from is different on CPT that on every other chip. We fixed that, but didn't (foolishly) review the non-DP code.
Oddly, the non-DP code had essentially the same bug:
[PATCH 3/4] drm/i915: Fix PCH port pipe select in CPT disable paths
Also in this sequence is some code that validates the LVDS panel power sequencing which discovered that the LVDS panel wasn't being disabled correctly because the LVDS registers were locked in the DPMS function. Instead of trying to get the LVDS locking right, I've just unlocked the registers in the lvds initialization code and left them unlocked after that:
[PATCH 1/4] drm/i915: Wait for LVDS panel power sequence [PATCH 2/4] drm/i915: Leave LVDS registers unlocked
Finally, I removed an unused argument to dp_pipe_enable while I was poking around in that code:
[PATCH 4/4] drm/i915: Remove unused 'reg' argument to dp_pipe_enable
-- keith.packard@intel.com