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 a4fc5ed698
Author: Keith Packard <keithp@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 e85194641b
Author: Adam Jackson <ajax@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@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51493
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
fba92150aa