[Intel-gfx] SNB LVDS goes all stripy at times with rc6 enabled

[Keith Packard]

On Wed, 27 Jul 2011 09:06:09 -0700, Jesse Barnes <jbarnes at virtuousgeek.org> wrote:

> I tried this last week with my patch

So, with the intel_crtc DPMS tracking fixed, this problem can now be
easily reproduced in a wide range of situations.

It does not depend on the output in question -- Jesse and I have managed
to make this happen on VGA, DP, LVDS and HDMI. The only thing we haven't
tested is eDP. That one would be particularly interesting as it doesn't
depend on the PCH. Anyone have an SNB machine with an eDP panel and an
external monitor to try?

It does not depend on the position of the outputs on the screen, all
that it requires is that one output be disabled and then the other
output get a new mode. Something like:

$ xrandr --output LVDS1 --pos 0x0 --mode 1024x768 --output DP2 --pos 0x0 --mode 1024x768
$ xrandr --output DP2 --off
$ xrandr --output LVDS1 --auto  # this assumes that LVDS1 is larger than 1024x768

The PCH reports no FDI data errors, and if you send the errant monitor
the PCH-internal test signal (bit 31, register 0xf0e30 and 0xf1e30), it
displays that correctly.

So, it surely looks like the CPU is sending the wrong data to the PCH in
this case.

At this point, I have only two proposed ugly work-arounds:

 1) Disable rc6.
 2) Once a crtc is turned on, leave it on.

Neither of these is appealing for obvious reasons.

At this point, it seems like the most likely cause is in the crtc
disabling code; some ordering issue or missing pieces may be scrambling
some hidden internal state.

-- 
keith.packard at intel.com
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