diff --git a/src/surface.c b/src/surface.c
index 44b4325..341637c 100644
--- a/src/surface.c
+++ b/src/surface.c
@@ -384,10 +384,87 @@ VAStatus RequestSyncSurface(VADriverContextP context, VASurfaceID surface_id)
 	 * (→ EXPORTED), the next BeginPicture for this surface (slot is
 	 * released first), or DestroySurfaces (release).
 	 */
-	if (surface_object->current_slot != NULL)
+	if (surface_object->current_slot != NULL) {
 		cap_pool_mark_decoded(&driver_data->capture_pool,
 				      surface_object->current_slot);
 
+		/*
+		 * iter8 Phase 6 (γ): env-gated diagnostic dump of the CAPTURE
+		 * buffer immediately after DQBUF + mark_decoded. Distinguishes
+		 * "kernel didn't write" from "libva mis-reads" for Bug 4
+		 * (H.264 partial-fill). Off by default; enable with
+		 * LIBVA_V4L2_DUMP_CAPTURE=1. destination_data[] is valid here
+		 * (surface_bind_slot populated it at BeginPicture).
+		 */
+		static const char *dump_env = NULL;
+		static bool dump_env_checked = false;
+		if (!dump_env_checked) {
+			dump_env = getenv("LIBVA_V4L2_DUMP_CAPTURE");
+			dump_env_checked = true;
+		}
+		if (dump_env != NULL && dump_env[0] == '1') {
+			unsigned int p;
+			char hexbuf[128];
+			request_log("γ-dump: surface_id=%u v4l2_index=%u planes=%u\n",
+				    (unsigned int)surface_id,
+				    surface_object->destination_index,
+				    surface_object->destination_planes_count);
+			for (p = 0; p < surface_object->destination_planes_count; p++) {
+				const unsigned char *d = surface_object->destination_data[p];
+				size_t sz = surface_object->destination_sizes[p];
+				size_t scan_lim;
+				unsigned int nz = 0;
+				size_t i;
+				int pos;
+
+				if (d == NULL) {
+					request_log("γ-dump:  plane[%u] NULL ptr (size=%zu)\n",
+						    p, sz);
+					continue;
+				}
+
+				/*
+				 * Phase 5 MIN-2: scan at least one Y-MB row
+				 * (16 lines * bytesperline) for plane 0, else
+				 * 1024 bytes for chroma plane.
+				 */
+				if (p == 0) {
+					size_t mbrow =
+					    surface_object->destination_bytesperlines[0] * 16;
+					scan_lim = sz < mbrow ? sz : mbrow;
+				} else {
+					scan_lim = sz < 1024 ? sz : 1024;
+				}
+				for (i = 0; i < scan_lim; i++)
+					if (d[i] != 0)
+						nz++;
+
+				request_log("γ-dump:  plane[%u] sz=%zu bpl=%u "
+					    "scan=%zu non_zero=%u\n",
+					    p, sz,
+					    surface_object->destination_bytesperlines[p],
+					    scan_lim, nz);
+
+				pos = 0;
+				for (i = 0; i < 32 && i < sz; i++)
+					pos += snprintf(hexbuf + pos,
+							sizeof(hexbuf) - pos,
+							"%02x ", d[i]);
+				request_log("γ-dump:  plane[%u] head[0..32]: %s\n",
+					    p, hexbuf);
+
+				if (sz >= 32) {
+					pos = 0;
+					for (i = 0; i < 32; i++)
+						pos += snprintf(hexbuf + pos,
+								sizeof(hexbuf) - pos,
+								"%02x ", d[sz - 32 + i]);
+					request_log("γ-dump:  plane[%u] tail[%zu..%zu]: %s\n",
+						    p, sz - 32, sz - 1, hexbuf);
+				}
+			}
+		}
+	}
 
 	surface_object->status = VASurfaceDisplaying;