[vpr][utils] add comment for get_atom_pin_rr_node_id

Author: amin1377

vpr/src/util/vpr_utils.cpp

@@ -1726,6 +1726,15 @@ RRNodeId get_atom_pin_rr_node_id(const AtomPinId& atom_pin_id) {
     auto& place_ctx = g_vpr_ctx.placement();
     auto& device_ctx = g_vpr_ctx.device();
 
+    /*
+     * To get the RRNodeId for an atom pin, we need to:
+     * 1. Find the atom block that the pin belongs to
+     * 2. Find the cluster block that the atom block is a part of
+     * 3. Find the physical tile that the cluster block is located on
+     * 4. Find the physical pin number of the atom pin (corresponds to ptc number of the RR node)
+     * 5. Call get_pin_rr_node_id to get the RRNodeId for the pin
+     */
+
     AtomBlockId atom_blk_id = atom_nlist.pin_block(atom_pin_id);
     ClusterBlockId clb_blk_id = atom_lookup.atom_clb(atom_blk_id);
 

vpr/src/util/vpr_utils.h

@@ -248,6 +248,12 @@ RRNodeId get_pin_rr_node_id(const RRSpatialLookup& rr_spatial_lookup,
                             const int root_j,
                             int pin_physical_num);
 
+/**
+ * @brief Returns the RR node ID for the given atom pin ID.
+ * **Warning**: This function should be called only if flat-router is enabled,
+ * since, otherwise, the routing resources inside clusters are not added to the RR graph.
+ * @param atom_pin_id The atom pin ID.
+ */
 RRNodeId get_atom_pin_rr_node_id(const AtomPinId& atom_pin_id);
 
 RRNodeId get_class_rr_node_id(const RRSpatialLookup& rr_spatial_lookup,