Merge pull request #2637 from Pinata-Consulting/docs-ROUTING_LAYER_ADJUSTMENT-color

docs: add some ROUTING_LAYER_ADJUSTMENT color

Author: maliberty

flow/scripts/variables.yaml

@@ -42,7 +42,32 @@ TNS_END_PERCENT:
     - grt
 ROUTING_LAYER_ADJUSTMENT:
   default: 0.5
-  description: Default routing layer adjustment
+  description: >
+    Adjusts routing layer capacities to manage congestion and
+    improve detailed routing. High values ease detailed routing
+    but risk excessive detours and long global routing times,
+    while low values reduce global routing failure but can
+    complicate detailed routing.
+
+    The global routing running time normally reduces
+    dramatically(entirely design specific, but going from hours to
+    minutes has been observed) when the value is
+    low(such as 0.10).
+
+    Sometimes, global routing will succeed with lower values and
+    fail with higher values. Exploring results with different
+    values can help shed light on the problem. Start with
+    a too low value, such as 0.10, and bisect
+    to value that works by doing multiple global routing runs.
+
+    As a last resort, `make global_route_issue` and using
+    the tools/OpenROAD/etc/deltaDebug.py can be useful to debug
+    global routing errors. If there is something specific that is
+    impossible to route, such as a clock line over a macro, global
+    routing will terminate with DRC errors routes that could
+    have been routed were it not for the specific impossible routes.
+    deltaDebug.py should weed out the possible routes and leave
+    a minimal failing case that pinpoints the problem.
   stages:
     - place
     - grt