feat(precheck): add warning for flattened cells

Flattened sky130 devices in analog layouts can lead to DRC errors.
They are difficult to test for, so as a heuristic we add a warning when magic DRC fails _and_ there are no unflattened sky130 cells.

precheck/precheck.py

@@ -10,6 +10,7 @@
 import klayout.db as pya
 import klayout.rdb as rdb
 from klayout_tools import parse_lyp_layers
+import gdstk
 
 PDK_ROOT = os.getenv("PDK_ROOT")
 PDK_NAME = os.getenv("PDK_NAME") or "sky130A"
@@ -25,6 +26,13 @@ class PrecheckFailure(Exception):
     pass
 
 
+def has_sky130_devices(gds: str):
+    for cell_name in gdstk.read_rawcells(gds):
+        if cell_name.startswith('sky130_fd_'):
+            return True
+    return False
+
+
 def magic_drc(gds: str, toplevel: str):
     logging.info(f"Running magic DRC on {gds} (module={toplevel})")
 
@@ -45,6 +53,8 @@ def magic_drc(gds: str, toplevel: str):
     )
 
     if magic.returncode != 0:
+        if not has_sky130_devices(gds):
+            logging.warning("No sky130 devices present - was the design flattened?")
         raise PrecheckFailure("Magic DRC failed")