commit c4ac230f486cc31fdb303777e23049910c92df74
Author: Michel Dänzer <michel.daenzer@amd.com>
Date:   Wed Oct 22 18:41:07 2014 +0900

    Add some debugging output about why page flipping isn't possible

diff --git a/src/radeon_dri2.c b/src/radeon_dri2.c
index 93ed950..2d3740c 100644
--- a/src/radeon_dri2.c
+++ b/src/radeon_dri2.c
@@ -717,25 +717,40 @@ can_exchange(ScrnInfoPtr pScrn, DrawablePtr draw,
     for (i = 0; i < xf86_config->num_crtc; i++) {
 	xf86CrtcPtr crtc = xf86_config->crtc[i];
 	if (crtc->enabled && crtc->rotatedData)
+	    ErrorF("Can't exchange because CRTC %d is rotated\n", i);
 	    return FALSE;
     }
 
-    if (!update_front(draw, front))
+    if (!update_front(draw, front)) {
+	ErrorF("Can't exchange because update_front failed\n");
 	return FALSE;
+    }
 
     front_pixmap = front_priv->pixmap;
 
-    if (front_pixmap->drawable.width != back_pixmap->drawable.width)
+    if (front_pixmap->drawable.width != back_pixmap->drawable.width) {
+	ErrorF("Can't exchange because pixmap widths don't match: %u != %u\n",
+	       front_pixmap->drawable.width, back_pixmap->drawable.width);
 	return FALSE;
+    }
 
-    if (front_pixmap->drawable.height != back_pixmap->drawable.height)
+    if (front_pixmap->drawable.height != back_pixmap->drawable.height) {
+	ErrorF("Can't exchange because pixmap heights don't match: %u != %u\n",
+	       front_pixmap->drawable.height, back_pixmap->drawable.height);
 	return FALSE;
+    }
 
-    if (front_pixmap->drawable.bitsPerPixel != back_pixmap->drawable.bitsPerPixel)
+    if (front_pixmap->drawable.bitsPerPixel != back_pixmap->drawable.bitsPerPixel) {
+	ErrorF("Can't exchange because pixmap bpps don't match: %u != %u\n",
+	       front_pixmap->drawable.bitsPerPixel, back_pixmap->drawable.bitsPerPixel);
 	return FALSE;
+    }
 
-    if (front_pixmap->devKind != back_pixmap->devKind)
+    if (front_pixmap->devKind != back_pixmap->devKind) {
+	ErrorF("Can't exchange because pixmap pitches don't match: %u != %u\n",
+	       front_pixmap->devKind, back_pixmap->devKind);
 	return FALSE;
+    }
 
     return TRUE;
 }
@@ -744,11 +759,32 @@ static Bool
 can_flip(ScrnInfoPtr pScrn, DrawablePtr draw,
 	 DRI2BufferPtr front, DRI2BufferPtr back)
 {
-    return draw->type == DRAWABLE_WINDOW &&
-	   RADEONPTR(pScrn)->allowPageFlip &&
-	   pScrn->vtSema &&
-	   DRI2CanFlip(draw) &&
-	   can_exchange(pScrn, draw, front, back);
+    if (draw->type != DRAWABLE_WINDOW) {
+	ErrorF("Can't flip because drawable isn't a window\n");
+	return FALSE;
+    }
+
+    if (!RADEONPTR(pScrn)->allowPageFlip) {
+	ErrorF("Can't flip because Option \"EnablePageFlip\" is disabled\n");
+	return FALSE;
+    }
+
+    if (!pScrn->vtSema) {
+	ErrorF("Can't flip because pScrn->vtSema is FALSE\n");
+	return FALSE;
+    }
+
+    if (!DRI2CanFlip(draw)) {
+	ErrorF("Can't flip because DRI2CanFlip failed\n");
+	return FALSE;
+    }
+
+    if (!can_exchange(pScrn, draw, front, back)) {
+	ErrorF("Can't flip because can_exchange failed\n");
+	return FALSE;
+    }
+
+    return TRUE;
 }
 
 static void