gt2n: set MAX_ROUTING_LAYER per design instead of platform-wide

The platform exposes metal up to M13, but the reference designs route well
below that (gcd uses up to M5, jpeg M9, aes M10; layers above carry no wire).
Pinning MAX_ROUTING_LAYER at the platform M13 top forced detailed routing to
build its grid graph, via stack, and DRC checks across the full layer range
for every design, inflating runtime and memory.

Move MAX_ROUTING_LAYER out of the platform config (leaving a comment) and set
it per design at the highest layer each actually uses. gcd@M5 was verified to
route DRC-clean; aes/jpeg caps only drop layers that carry zero wire.

Signed-off-by: mrg <mrg@ucsc.edu>

Author: mguthaus

flow/designs/gt2n/aes/config.mk

@@ -17,3 +17,8 @@ export SKIP_INCREMENTAL_REPAIR = 1
 
 export SWAP_ARITH_OPERATORS = 1
 export OPENROAD_HIERARCHICAL = 1
+
+# Highest layer aes actually routes on (M11-M13 carry no wire). Capping here
+# instead of the platform M13 top shrinks the detailed-route grid, vias, and
+# DRC. See platforms/gt2n/config.mk.
+export MAX_ROUTING_LAYER = M10

flow/designs/gt2n/gcd/config.mk

@@ -9,3 +9,8 @@ export CORE_UTILIZATION   = 25
 export CORE_MARGIN        = 0.5
 export PLACE_DENSITY      = 0.35
 export TNS_END_PERCENT    = 100
+
+# Highest layer gcd actually routes on (M6-M13 carry no wire). Capping here
+# instead of the platform M13 top shrinks the detailed-route grid, vias, and
+# DRC. See platforms/gt2n/config.mk.
+export MAX_ROUTING_LAYER  = M5

flow/designs/gt2n/jpeg/config.mk

@@ -14,3 +14,8 @@ export CORE_ASPECT_RATIO      = 1
 export CORE_MARGIN            = 2
 export PLACE_DENSITY_LB_ADDON = 0.20
 export TNS_END_PERCENT        = 100
+
+# Highest layer jpeg actually routes on (M10-M13 carry no wire). Capping here
+# instead of the platform M13 top shrinks the detailed-route grid, vias, and
+# DRC. See platforms/gt2n/config.mk.
+export MAX_ROUTING_LAYER      = M9

flow/platforms/gt2n/config.mk

@@ -118,7 +118,12 @@ export PLACE_DENSITY ?= 0.40
 #---------------------------------------------------------
 export MIN_ROUTING_LAYER = M2
 export MIN_CLK_ROUTING_LAYER = M3
-export MAX_ROUTING_LAYER = M13
+# MAX_ROUTING_LAYER is set per design (designs/gt2n/<design>/config.mk), not
+# platform-wide. gt2n exposes metal up to M13, but designs route well below
+# that (e.g. gcd~M5, jpeg~M9, aes~M10); routing to M13 needlessly grows the
+# detailed-route grid graph, via stack, and DRC work for layers that carry no
+# wire. Each gt2n design must define MAX_ROUTING_LAYER (the flow requires it)
+# at the highest layer it actually needs.
 
 export FASTROUTE_TCL ?= $(PLATFORM_DIR)/fastroute.tcl