rfnoc: Modify prop. propagation algorithm (back-edge resolution)

Previously, the property propagation algorithm would first forward and
resolve properties only along forward edges. Then, we would check that
properties also align across back-edges. The assumption is that graphs
are always structured in a way such that back-edges would align when the
resolution is done.

However, for the following graph, this would fail:

Radio ---> Replay
  ^         |
  +---------+

The reason is that the radio block and the replay block both have an
"atomic_item_size" property, which needs to be resolved both ways. If
the default atomic_item_size is 4 for the radio, and 8 for the replay
block, then the input atomic_item_size on the radio will never be
aligned with the output atomic_item_size of the replay block, and there
is no other mechanism to align those.

The solution is to run the edge property propagation and resolution
twice, first for the forward edges, then for the back-edges. For graphs
that would previously work, this makes no difference: The additional
step of propagation properties across the back-edges will not dirty any
properties. However, for graphs like the one above, it will provide an
additional resolution path for properties that are otherwise not
connected.

1 files changed, 3 insertions, 2 deletions

diff --git a/host/tests/rfnoc_propprop_test.cpp b/host/tests/rfnoc_propprop_test.cpp
index 1752e4c21..130137a28 100644
--- a/host/tests/rfnoc_propprop_test.cpp
+++ b/host/tests/rfnoc_propprop_test.cpp
@@ -385,8 +385,9 @@ BOOST_AUTO_TEST_CASE(test_graph_ro_prop)
     const size_t rx_rssi_resolver_count = mock_rx_radio.rssi_resolver_count;
     UHD_LOG_INFO("TEST", "Now testing mock RSSI resolver/get prop");
     UHD_LOG_DEBUG("TEST", "RX RSSI: " << mock_rx_radio.get_property<double>("rssi"));
-    // The next value must match the value in graph.cpp
-    BOOST_CHECK_EQUAL(rx_rssi_resolver_count + 1, mock_rx_radio.rssi_resolver_count);
+    // The next value must match the value in graph.cpp. We have one additional
+    // backward- and forward resolution.
+    BOOST_CHECK_EQUAL(rx_rssi_resolver_count + 2, mock_rx_radio.rssi_resolver_count);
 }
 
 BOOST_AUTO_TEST_CASE(test_graph_double_connect)