Re: [PATCH 5/9] drm/i915: CPT/PPT pch dp transcoder workaround

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Oct 26, 2012 at 12:21:11PM -0200, Paulo Zanoni wrote:
> Hi
> 
> 2012/10/26 Daniel Vetter <daniel.vetter@ffwll.ch>:
> > We need to set the timing override chicken bit after fdi link training
> > has completed and before we enable the dp transcoder. We also have to
> > clear that bit again after disabling the pch dp transcoder.
> >
> > See "Graphics BSpec: vol4g North Display Engine Registers [IVB],
> > Display Mode Set Sequence" and "Graphics BSpec: vol4h South Display
> > Engine Registers [CPT, PPT], South Display Engine Transcoder and FDI
> > Control, Transcoder Debug and DFT, TRANS_CHICKEN_2" bit 31:
> >
> > "Workaround : Enable the override prior to enabling the transcoder.
> > Disable the override after disabling the transcoder."
> >
> > While at it, use the _PIPE macro for the other TRANS_DP register.
> 
> What confuses me is that the bit name is actually "Autotrain TimingGen
> Override" and after a quick check it seems we don't use autotrain.
> Also, by looking at the IVB mode set sequence it seems Auto Train is
> the *recommended* method since manual is listed as "testing only". So
> I'm not sure if we need this code now, but it seems we could try to
> implement the auto train...

Well, us using manual fdi train is a different matter altogether. And
since there's nothing in the docs indicating that we don't need this w/a
if we don't do autotraining, I prefer to just stick it into the code.

And Damien tried to do the auto training, but somehow it failed ...

> Also, I'd move the code that sets/unsets the chicken bit to
> intel_enable_transcoder/intel_disable_transcoder.

According to my reading of the modeset sequence we need to do this after
the fdi train is fully done, but before we enable the pch dp transcoder.
So I think this is the right place, and I can't actually move it anyplace
else. And since the pch dp transcoder is a cpt/ppt-only feature, it' fits
rather naturally.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch