drm/vc4: Enable precise vblank timestamping for interlaced modes.

On top of the interlaced video mode fix and with some additional
adjustments, this now works well. It has almost the same accuracy
as on regular progressive scan modes.
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/vc4/vc4_crtc.c

@@ -163,14 +163,6 @@ int vc4_crtc_get_scanoutpos(struct drm_device *dev, unsigned int crtc_id,
 	int vblank_lines;
 	int ret = 0;
 
-	/*
-	 * XXX Doesn't work well in interlaced mode yet, partially due
-	 * to problems in vc4 kms or drm core interlaced mode handling,
-	 * so disable for now in interlaced mode.
-	 */
-	if (mode->flags & DRM_MODE_FLAG_INTERLACE)
-		return ret;
-
 	/* preempt_disable_rt() should go right here in PREEMPT_RT patchset. */
 
 	/* Get optional system timestamp before query. */
@@ -191,11 +183,16 @@ int vc4_crtc_get_scanoutpos(struct drm_device *dev, unsigned int crtc_id,
 
 	/* Vertical position of hvs composed scanline. */
 	*vpos = VC4_GET_FIELD(val, SCALER_DISPSTATX_LINE);
-
-	/* No hpos info available. */
-	if (hpos)
 	*hpos = 0;
 
+	if (mode->flags & DRM_MODE_FLAG_INTERLACE) {
+		*vpos /= 2;
+
+		/* Use hpos to correct for field offset in interlaced mode. */
+		if (VC4_GET_FIELD(val, SCALER_DISPSTATX_FRAME_COUNT) % 2)
+			*hpos += mode->crtc_htotal / 2;
+	}
+
 	/* This is the offset we need for translating hvs -> pv scanout pos. */
 	fifo_lines = vc4_crtc->cob_size / mode->crtc_hdisplay;
 
@@ -217,8 +214,6 @@ int vc4_crtc_get_scanoutpos(struct drm_device *dev, unsigned int crtc_id,
 		 * position of the PV.
 		 */
 		*vpos -= fifo_lines + 1;
-		if (mode->flags & DRM_MODE_FLAG_INTERLACE)
-			*vpos /= 2;
 
 		ret |= DRM_SCANOUTPOS_ACCURATE;
 		return ret;