rfnoc: Fix issue in uhd::rfnoc::connect_through_blocks()

When connect_through_blocks() was called on blocks within a single
chain, there was a bug where the chain was incorrectly cropped. In
a standard FPGA image, say one was to use this API call to connect the
radio to the DDC. It would generate a chain of blocks hanging off the
radio as such:

Radio -> DDC -> SEP

What the code should do, and what this fix provides, is that the chain
gets cropped after the DDC, to look like this:

Radio -> DDC

With the current bug, it would assume the chain has a dangling edge, and
incorrectly throw an exception.
Note that this bug would not appear when source and destination block
are on separate chains (i.e., both have an SEP in their chain).

This patch includes minor logging and comment improvements around the
offending lines of code.

1 files changed, 9 insertions, 6 deletions

diff --git a/host/lib/utils/graph_utils.cpp b/host/lib/utils/graph_utils.cpp
index f21000ccf..513f7b709 100644
--- a/host/lib/utils/graph_utils.cpp
+++ b/host/lib/utils/graph_utils.cpp
@@ -56,7 +56,7 @@ std::vector<graph_edge_t> get_block_chain(const rfnoc_graph::sptr graph,
                 // If the current block is the edge's source, make the edge's
                 // destination the current block
                 next_found = true;
-                UHD_LOG_TRACE("GRAPH_UTILS", "Found next block: " + edge.dst_blockid);
+                UHD_LOG_TRACE("GRAPH_UTILS", " --> Found next block: " + edge.dst_blockid);
 
                 block_chain.push_back(edge);
                 current_block = (source_chain) ? edge.dst_blockid : edge.src_blockid;
@@ -103,12 +103,14 @@ void connect_through_blocks(rfnoc_graph::sptr graph,
                    || (dst_blk.to_string() == edge.dst_blockid
                        && dst_port == edge.dst_port);
         });
-    // If our dst_blk is in the chain already, make sure its the last element and continue
     if (dst_found) {
+        // If our dst_blk is in the chain already, make sure it's the last element
+        // and continue. This means we pop everything from block_chain that comes
+        // after our block.
         UHD_LOG_TRACE(
             "GRAPH_UTILS", "Found dst_blk (" + dst_blk.to_string() + ") in source chain");
-        while (dst_blk.to_string() == block_chain.back().dst_blockid
-               && dst_port == block_chain.back().dst_port) {
+        while (dst_blk.to_string() != block_chain.back().dst_blockid
+               || dst_port != block_chain.back().dst_port) {
             UHD_LOG_TRACE("GRAPH_UTILS",
                 boost::format(
                     "Last block (%s:%d) doesn't match dst_blk (%s:%d); removing.")
@@ -153,8 +155,9 @@ void connect_through_blocks(rfnoc_graph::sptr graph,
     if (has_src_to_sep_connection && has_sep_to_dst_connection) {
         graph->connect(src_to_sep_id, src_to_sep_port, sep_to_dst_id, sep_to_dst_port);
     } else if (has_src_to_sep_connection != has_sep_to_dst_connection) {
-        throw uhd::runtime_error(
-            "[graph_utils] Incomplete path. Only one SEP edge found.");
+        const std::string err_msg = "Incomplete path. Only one SEP edge found.";
+        UHD_LOG_TRACE("GRAPH_UTILS", err_msg);
+        throw uhd::runtime_error("[graph_utils] " + err_msg);
     }
 }