drm/i915: Fix scanoutpos calculations

The reported scanout position must be relative to the end of vblank.
Currently we manage to fumble that in a few ways.

First we don't consider the case when vtotal != vbl_end. While that
isn't very common (happens maybe only w/ old panel fitting hardware),
we can fix it easily enough.

The second issue is that on pre-CTG hardware we convert the pixel count
to horizontal/vertical components at the very beginning, and then forget
to adjust the horizontal component to be relative to vbl_end. So instead
we should keep our numbers in the pixel count domain while we're
adjusting the position to be relative to vbl_end. Then when we do the
conversion in the end, both vertical _and_ horizontal components will
come out correct.

v2: Change position to int from u32 to avoid sign issues

Cc: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: mario.kleiner.de@gmail.com
Tested-by: mario.kleiner.de@gmail.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -599,7 +599,7 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
 	struct drm_crtc *crtc = dev_priv->pipe_to_crtc_mapping[pipe];
 	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
 	const struct drm_display_mode *mode = &intel_crtc->config.adjusted_mode;
-	u32 position;
+	int position;
 	int vbl_start, vbl_end, htotal, vtotal;
 	bool in_vbl = true;
 	int ret = 0;
@@ -621,13 +621,7 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
 		/* No obvious pixelcount register. Only query vertical
 		 * scanout position from Display scan line register.
 		 */
-		position = I915_READ(PIPEDSL(pipe));
-
-		/* Decode into vertical scanout position. Don't have
-		 * horizontal scanout position.
-		 */
-		*vpos = position & 0x1fff;
-		*hpos = 0;
+		position = I915_READ(PIPEDSL(pipe)) & 0x1fff;
 	} else {
 		/* Have access to pixelcount since start of frame.
 		 * We can split this into vertical and horizontal
@@ -635,15 +629,32 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
 		 */
 		position = (I915_READ(PIPEFRAMEPIXEL(pipe)) & PIPE_PIXEL_MASK) >> PIPE_PIXEL_SHIFT;
 
-		*vpos = position / htotal;
-		*hpos = position - (*vpos * htotal);
+		/* convert to pixel counts */
+		vbl_start *= htotal;
+		vbl_end *= htotal;
+		vtotal *= htotal;
 	}
 
-	in_vbl = *vpos >= vbl_start && *vpos < vbl_end;
+	in_vbl = position >= vbl_start && position < vbl_end;
+
+	/*
+	 * While in vblank, position will be negative
+	 * counting up towards 0 at vbl_end. And outside
+	 * vblank, position will be positive counting
+	 * up since vbl_end.
+	 */
+	if (position >= vbl_start)
+		position -= vbl_end;
+	else
+		position += vtotal - vbl_end;
 
-	/* Inside "upper part" of vblank area? Apply corrective offset: */
-	if (in_vbl && (*vpos >= vbl_start))
-		*vpos = *vpos - vtotal;
+	if (IS_G4X(dev) || INTEL_INFO(dev)->gen >= 5) {
+		*vpos = position;
+		*hpos = 0;
+	} else {
+		*vpos = position / htotal;
+		*hpos = position - (*vpos * htotal);
+	}
 
 	/* In vblank? */
 	if (in_vbl)