From 56587859d3596c4a42d5f28ae51898ee09cea2c1 Mon Sep 17 00:00:00 2001
From: gatecat <gatecat@ds0.me>
Date: Mon, 18 Dec 2023 12:18:22 +0000
Subject: [PATCH] nexus: Improve error reporting for illegal carry chains

Signed-off-by: gatecat <gatecat@ds0.me>
---
 nexus/pack.cc | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/nexus/pack.cc b/nexus/pack.cc
index d5c7147e..dad3722a 100644
--- a/nexus/pack.cc
+++ b/nexus/pack.cc
@@ -1318,8 +1318,14 @@ struct NexusPacker
             CellInfo *ci = cell.second.get();
             if (ci->type != id_CCU2)
                 continue;
-            if (ci->getPort(id_CIN) != nullptr)
+            NetInfo *cin = ci->getPort(id_CIN);
+            if (cin) {
+                if (cin->driver.cell && cin->driver.cell->type != id_CCU2) {
+                    log_error("CCU2 '%s' CIN net '%s' driven by non-CCU2 cell '%s'.\n",
+                        ctx->nameOf(ci), ctx->nameOf(cin), ctx->nameOf(cin->driver.cell));
+                }
                 continue;
+            }
             roots.push_back(ci);
         }
         for (CellInfo *root : roots) {