drm/vblank: Enable precise vblank timestamps for interlaced and doublescan modes.

Testing showed the current code can already handle doublescan
video modes just fine. A trivial tweak makes it work for interlaced
scanout as well.

Tested and shown to be precise on Radeon rv530, r600 and
Intel 945-GME.
Signed-off-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_irq.c

@@ -493,6 +493,12 @@ void drm_calc_timestamping_constants(struct drm_crtc *crtc)
 	/* Dot clock in Hz: */
 	dotclock = (u64) crtc->hwmode.clock * 1000;
 
+	/* Fields of interlaced scanout modes are only halve a frame duration.
+	 * Double the dotclock to get halve the frame-/line-/pixelduration.
+	 */
+	if (crtc->hwmode.flags & DRM_MODE_FLAG_INTERLACE)
+		dotclock *= 2;
+
 	/* Valid dotclock? */
 	if (dotclock > 0) {
 		/* Convert scanline length in pixels and video dot clock to
@@ -605,14 +611,6 @@ int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev, int crtc,
 		return -EAGAIN;
 	}
 
-	/* Don't know yet how to handle interlaced or
-	 * double scan modes. Just no-op for now.
-	 */
-	if (mode->flags & (DRM_MODE_FLAG_INTERLACE | DRM_MODE_FLAG_DBLSCAN)) {
-		DRM_DEBUG("crtc %d: Noop due to unsupported mode.\n", crtc);
-		return -ENOTSUPP;
-	}
-
 	/* Get current scanout position with system timestamp.
 	 * Repeat query up to DRM_TIMESTAMP_MAXRETRIES times
 	 * if single query takes longer than max_error nanoseconds.