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>
---
drivers/gpu/drm/i915/intel_dp.c | 13 -------------
1 file changed, 13 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 376ae28..3a4ebd4 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -1778,25 +1778,12 @@ static void
intel_dp_start_link_train(struct intel_dp *intel_dp)
{
struct drm_device *dev = intel_dp->base.base.dev;
- struct drm_i915_private *dev_priv = dev->dev_private;
- struct intel_crtc *intel_crtc = to_intel_crtc(intel_dp->base.base.crtc);
int i;
uint8_t voltage;
bool clock_recovery = false;
int voltage_tries, loop_tries;
uint32_t DP = intel_dp->DP;
- /*
- * On CPT we have to enable the port in training pattern 1, which
- * will happen below in intel_dp_set_link_train. Otherwise, enable
- * the port and wait for it to become active.
- */
- if (!HAS_PCH_CPT(dev)) {
- I915_WRITE(intel_dp->output_reg, intel_dp->DP);
- POSTING_READ(intel_dp->output_reg);
- intel_wait_for_vblank(dev, intel_crtc->pipe);
- }
-
/* Write the link configuration data */
intel_dp_aux_native_write(intel_dp, DP_LINK_BW_SET,
intel_dp->link_configuration,
--
1.7.10.4
