[Intel-gfx] [PATCH] drm/i915: Add a workaround for HSW scanline counter weirdness

[ville.syrjala at linux.intel.com]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

On HSW the scanline counter seems to behave differently depending on
the output type. eDP on port A does what you would expect an the normal
+1 fixup is sufficient to cover it. But on HDMI outputs we seem to need
a +2 fixup. Just assume we always need the +2 fixup and accept the
slight inaccuracy on eDP.

This fixes a regression introduced in:
 commit 8072bfa6045a264d3913102a35fab125b06603a2
 Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
 Date:   Mon Oct 28 21:22:52 2013 +0200

    drm/radeon: Move the early vblank IRQ fixup to radeon_get_crtc_scanoutpos()

That commit removed the heuristic that tried to fix up the timestamps
for vblank interrupts that fire a bit too early. Since then the vblank
timestamp code would treat some vblank interrupts as spurious since the
scanline counter would indicate that vblank_start wasn't reached yet.
That in turn lead to incorrect vblank event sequence numbers being
reported to userspace, which lead to unsteady framerate in applications
such as XBMC which uses them for timing purposes.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75725
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/i915/i915_irq.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 9fec711..72fce3d 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -702,7 +702,27 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
 		else
 			position = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) & DSL_LINEMASK_GEN3;
 
-		if (HAS_PCH_SPLIT(dev)) {
+		if (HAS_DDI(dev)) {
+			/*
+			 * On HSW HDMI outputs there seems to be a 2 line
+			 * difference, whereas eDP has the normal 1 line
+			 * difference that earlier platforms have. External
+			 * DP is unknown. For now just check for the 2 line
+			 * difference case on all output types on HSW+.
+			 *
+			 * This might misinterpret the scanline counter being
+			 * one line too far along on eDP, but that's less
+			 * dangerous than the alternative since that would lead
+			 * the vblank timestamp code astray when it sees a
+			 * scanline count before vblank_start during a vblank
+			 * interrupt.
+			 */
+			if ((in_vbl && (position == vbl_start - 2 ||
+					position == vbl_start - 1)) ||
+			    (!in_vbl && (position == vbl_end - 2 ||
+					 position == vbl_end - 1)))
+				position = (position + 2) % vtotal;
+		} else if (HAS_PCH_SPLIT(dev)) {
 			/*
 			 * The scanline counter increments at the leading edge
 			 * of hsync, ie. it completely misses the active portion
-- 
1.8.3.2