diff --git a/src/r6xx_accel.c b/src/r6xx_accel.c
index 752165b..ea7e85e 100644
--- a/src/r6xx_accel.c
+++ b/src/r6xx_accel.c
@@ -367,8 +367,10 @@ r600_cp_wait_vline_sync(ScrnInfoPtr pScrn, drmBufPtr ib, PixmapPtr pPix,
 	    return;
     }
 
+    ErrorF("(start, stop) before clamping: (%d, %d)\n", start, stop);
     start = max(start, 0);
     stop = min(stop, crtc->mode.VDisplay);
+    ErrorF("(start, stop) after clamping: (%d, %d), crtc->mode.VDisplay=%d\n", start, stop, crtc->mode.VDisplay);
 
     if (start > crtc->mode.VDisplay)
         return;
@@ -376,6 +378,7 @@ r600_cp_wait_vline_sync(ScrnInfoPtr pScrn, drmBufPtr ib, PixmapPtr pPix,
     /* on r5xx+ vline starts at viewport_y */
     start += crtc->y;
     stop += crtc->y;
+    ErrorF("(start, stop) after translation: (%d, %d)\n", start, stop);
 
 #if defined(XF86DRM_MODE)
     if (info->cs) {