[PATCH v2] drm/i915: Fix shifted screen on top of LVDS on IVY laptop

[Takashi Iwai <tiwai@suse.de>]

The commit [23670b322: drm/i915: CPT+ pch transcoder workaround]
caused a regression on some HP laptops with IvyBridge.  The whole
laptop screen is shifted downward for a few pixels constantly.
The problem appears only on LVDS while DP and VGA seem unaffected.
Also, the problem disappears once when go and back from S3.
(S4 resume still shows the same problem.)

This patch revives the minimum part the commit above dropped.
For fixing this regression, only the setup of CHICKEN2 bit in
cpt_init_clock_gating() is needed.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
v1->v2: subject changed, more comments and changelog

 drivers/gpu/drm/i915/intel_pm.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index 58c2f21..3e3021d 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -3446,6 +3446,11 @@ static void cpt_init_clock_gating(struct drm_device *dev)
 	I915_WRITE(SOUTH_DSPCLK_GATE_D, PCH_DPLSUNIT_CLOCK_GATE_DISABLE);
 	I915_WRITE(SOUTH_CHICKEN2, I915_READ(SOUTH_CHICKEN2) |
 		   DPLS_EDP_PPS_FIX_DIS);
+	/* The below fixes the weird display corruption, a few pixels shifted
+	 * downward, on (only) LVDS of some HP laptops with IVY.
+	 */
+	for_each_pipe(pipe)
+		I915_WRITE(TRANS_CHICKEN2(pipe), TRANS_CHICKEN2_TIMING_OVERRIDE);
 	/* WADP0ClockGatingDisable */
 	for_each_pipe(pipe) {
 		I915_WRITE(TRANS_CHICKEN1(pipe),
-- 
1.8.0.1