[Bug 49402] [snb dp dongle] DP-to-dual-HDMI (link_train issue?)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=49402

--- Comment #19 from Ville Syrjala <ville.syrjala at linux.intel.com> ---
(In reply to comment #18)
> On Tue, 16 Apr 2013 09:50:53 +0000
> bugzilla-daemon at freedesktop.org wrote:
> 
> > https://bugs.freedesktop.org/show_bug.cgi?id=49402
> > 
> > --- Comment #17 from Ville Syrjala <ville.syrjala at linux.intel.com> ---
> > I tried a few random things for this on IVB.
> > 
> > - tried to send the training pattern and training lane set with one aux write
> > instead of two.
> > - tried to disable enhanced framing
> > - tried to force some voltage swing/pre-emphasis values
> > - tried with 6 bpc so that we'd get 1.62 links speed instead of 2.7
> > - tried to send post cursor2 traning values in case that would trick the sink
> > into cooperation
> > 
> > Unfortunately none of that helped.
> > 
> > The BIOS produces a picture with the dongle, so I tried to compare some of the
> > register values. The main differences I saw were that the BIOS uses auto
> > training for FDI, some of the M/N values were different, and of course the BIOS
> > also has panel fitter active since it's using a 720x400 resolution.
> 
> The m/n values could definitely be a factor.  Maybe our DP values are
> slightly off and causing some skew in the dongle?

It seems you are right. I just tried to hardcode the M/N values to the ones
used by the BIOS, and now I get a picture on the screen. I'll just list the
values here and hope someone else will figure out what code changes are needed
to produce the working values ;)

TU = 64 for both
data M/N:
 BIOS: 62ae4e / 800000
 i915: 65a360 / 83d600
link M/N:
 BIOS: 83931 / 80000
 i915: 43c24 / 41eb0

The DP link training still "fails" the same way. It tries CR five times w/
vswing=0 and pre-emphasis=0 before giving up, and then goes on to the EQ phase
which succeeds immediately. But the CR failure doesn't seem to prevent it from
working.

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