[Intel-gfx] [PATCH] drm/i915: rip out early dp port write for gm45/ilk

[Daniel Vetter]

On Thu, Sep 13, 2012 at 11:10:14AM +0100, Chris Wilson wrote:
> On Wed, 12 Sep 2012 23:24:09 +0200, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> > It's bogus.
> > 
> > If I've followed the history of this piece of code correctly, i.e. the
> > initial register write with the following vblank wait, this goes all
> > the way back to the original enabling of DP support in
> > 
> > commit a4fc5ed69817c73e32571ad7837bb707f9890009
> > Author: Keith Packard <keithp at keithp.com>
> > Date:   Tue Apr 7 16:16:42 2009 -0700
> > 
> >     drm/i915: Add Display Port support
> > 
> > Unfortunately it seems to be nothing more than glorified duct-tape and
> > sometimes actively harmful. Adam Jackson noticed this for CPT
> > platforms with
> > 
> > commit e85194641bec56179dcf5e1704ce5c6bf30340c6
> > Author: Adam Jackson <ajax at redhat.com>
> > Date:   Thu Jul 21 17:48:38 2011 -0400
> > 
> >     drm/i915/dp: Don't turn CPT DP ports on too early
> > 
> > Unfortunately this kept the code around for ilk and gm45.
> > 
> > The specific failure case I'm seeing here is that after a dpms off/on
> > cycle we have the bits from the last link training (hopefully
> > successful link training) set in intel_dp->DP. This is requiered so
> > that complete_link_train can enable the port with the right tuning
> > values.
> > 
> > Unfortunately writing these again to the disabled port at dpms on time
> > kills the port somehow until it's disabled - dp link training fails in
> > an endless loop without this patch on my mobile ilk and gm45.
> > 
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > Signed-Off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> 
> Works for me, only time will time if I no longer get the occasional
> failure in link training, but it definitely fixes the issue with dpms
> off.
> 
> Tested-by: Chris Wilson <chris at chris-wilson.co.uk>
> Probably https://bugs.freedesktop.org/show_bug.cgi?id=51493

Queued for -next, thanks for testing - imo this patch is too risky for
-fixes at this point in the release cycle.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch