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, 15 insertions, 11 deletions

diff --git a/host/docs/properties.dox b/host/docs/properties.dox
index 7ab99da3e..a56a94462 100644
--- a/host/docs/properties.dox
+++ b/host/docs/properties.dox
@@ -345,24 +345,28 @@ Consider the following graph:
 In this application, we receive a signal, use the DDC block to correct a
 frequency offset digitally, and then apply some custom DSP before transmitting
 the signal again, using the same radio block. In order to create a valid graph
-that UHD can handle, one of the edges needs to be declared a back edge.
+that UHD can handle, one of the edges needs to be declared a back-edge.
 
-Edge properties are not propagated across back edges. However, they must still
-match up after property resolution is done. For example, if the DDC were to be
+When declaring an edge as a back-edge, this signals to UHD that this edge should
+not be used for viewing the graph as a directed, acyclic graph (DAG), which is
+important during property propagation. In the graph above, UHD would sort blocks
+topologically (Radio Block, DDC Block, Custom DSP Block) by ignoring back-edges.
+Then, properties are forwarded in topological order first across forward edges,
+then across back edges.
+
+This can lead to resolution errors.  For example, if the DDC were to be
 configured to decimate the sampling rate, and the custom DSP block would not
 correct for that, the output edge property `samp_rate` of the custom DSP block
 would be mismatched compared to the input edge property `samp_rate` of the radio.
-In that case, UHD will throw an exception when trying to resolve properties,
-even if it is declared a back-edge.
+This could either re-trigger a new cascade of property settings, which means
+the algorithm can't converge, and UHD would detect that and throw an exception.
+Or the radio block might not accept a sample rate update on its input port,
+which would also trigger an exception.
 
-Note that property propagation along edges is required for certain operations.
-Therefore, it is highly recommended to not declare edges as back-edges unless
+It is highly recommended to not declare edges as back-edges unless
 necessary. In the graph above, any of the three edges could have been declared
 a back-edge to result in a topologically valid graph that can still resolve its
-properties, but the one chosen is the most intuitive selection. If any *two*
-edges were declared back-edges, at least one of the DDC or Custom DSP blocks
-might cease to function, e.g., because it doesn't have access to the tick rate
-property.
+properties, but the one chosen is the most intuitive selection.
 
 \section props_unknown Handling unknown properties