Re: Thinkpad T420 and single/dual channel lvds

[Adam Jackson <ajax@redhat.com>]

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On Wed, 2012-03-14 at 17:44 +0100, Takashi Iwai wrote:
> At Wed, 14 Mar 2012 10:45:06 -0400,
> Adam Jackson wrote:
> > 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.
> 
> Coincidently, we hit the same issue with a HP laptop, and wondered how
> is the best way to fix.  I hoped BIOS could handle better,
> i.e. setting the power bits no matter whether the lid is opened or
> not.  But it doesn't set unless the lid is once opened.
> 
> (Interestingly, the bits remain even if you close the lid again before
>  booting.  Just opening once seems triggering the probing of LVDS
>  panel in BIOS and let it setting the right values.)
> 
> FWIW, when I check ironalke_crtc_mode_set(), the clock of the HD+
> LVDS mode (1600x900) is 107800 (refclk 120000), while a similar
> machine with a HD panel (1366x768) shows 76300.  So, I'm not sure
> whether 112MHz could be a right threshold.

Hm, fair point.

Would be interesting to compare the VBTs between the two boots.  The
lvds_fp_timing struct might actually have the data we want here
regardless of how the machine booted.

- ajax

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