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