Otherwise the hw will get confused and result in a black screen.
This regression has been most likely introduce in
commit 974b93315b2213b74a42a87e8a9d4fc8c0dbe90c
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date: Sun Sep 5 00:44:20 2010 +0100
drm/i915/tv: Poll for DAC state change
That commit replace the first msleep(20) with a busy-loop, but failed
to keep the 2nd msleep around. Later on we've replaced all these
msleep(20) by proper vblanks.
For reference also see the commit in xf86-video-intel:
commit 1142be53eb8d2ee8a9b60ace5d49f0ba27332275
Author: Jesse Barnes <jbarnes at hobbes.lan>
Date: Mon Jun 9 08:52:59 2008 -0700
Fix TV programming: add vblank wait after TV_CTL writes
Fxies FDO bug #14000; we need to wait for vblank after
writing TV_CTL or following "DPMS on" calls may not actually enable the output.
Launchpad: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/763688
Reported-and-Tested-by: Robert Lowery <rglowery at exemail.com.au>
Cc: Rodrigo Vivi <rodrigo.vivi at gmail.com>
Cc: stable at kernel.org
Signed-Off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
drivers/gpu/drm/i915/intel_tv.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/drivers/gpu/drm/i915/intel_tv.c b/drivers/gpu/drm/i915/intel_tv.c
index 3346612..7b2bcdd 100644
--- a/drivers/gpu/drm/i915/intel_tv.c
+++ b/drivers/gpu/drm/i915/intel_tv.c
@@ -1195,6 +1195,11 @@ intel_tv_detect_type(struct intel_tv *intel_tv,
I915_WRITE(TV_DAC, save_tv_dac & ~TVDAC_STATE_CHG_EN);
I915_WRITE(TV_CTL, save_tv_ctl);
+ POSTING_READ(TV_CTL);
+
+ intel_wait_for_vblank(intel_tv->base.base.dev,
+ to_intel_crtc(intel_tv->base.base.crtc)->pipe);
+
/* Restore interrupt config */
if (connector->polled & DRM_CONNECTOR_POLL_HPD) {
spin_lock_irqsave(&dev_priv->irq_lock, irqflags);
--
1.7.10
