[Intel-gfx] Thinkpad T420 and single/dual channel lvds

Adam Jackson ajax at redhat.com
Wed Mar 14 15:45:06 CET 2012

On Wed, 2012-03-14 at 13:37 +0100, Helge Bahmann wrote:
> Hi everyone,
> Booting a Thinkpad T420 with the lid closed results and opening it subsequently results in an 
> unusable picture on the panel (and no amount of resetting makes it usable): The even pixels show 
> the nominal content of the framebuffer (with the right half missing) whlie the odd pixels show a 
> red-/blueish flicker.
> A little bit of investigation revealed that with the lid closed on boot, the panel ends up driven in 
> lvds single channel mode, which the panel probably does not like. It appears that the i915 driver 
> relies on the BIOS setting up the PCH_LVDS register for dual channel and will otherwise not ever 
> consider switching.
> While the following hack makes my display work, it is quite obviously not the right thing to do -- 
> could you investigate if there is a "proper" way to determine whether the panel is supposed to be 
> driven dual channel?

Nice find!

There may or may not be a bit for this in the VBT in the BIOS.  But the
more reliably correct thing I suspect would be to just look at the
preferred mode for the panel and assume it's dual-link LVDS if the pixel
clock is >112MHz, since that's the crossover frequency.

- ajax
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