Add a heartbeat counter to check the kernel

lagarto-ox.nix

@@ -79,6 +79,15 @@
       # Avoid zstd as we don't have the tools in "cucu" machine
       compressor = "gzip";
       kernelModules = [ ];
+
+      # Write a counter to the DMA region, so we can check the kernel is not
+      # dead. Monitor from the host with:
+      # while [ 1 ]; do xxd -s $((0x1bfff0000 - 0x60000000)) \
+      #   -l 4 /dev/qdma34000-MM-1; sleep 0.2; done
+      preDeviceCommands = ''
+        echo "Creating a heartbeat counter at 0x1bfff0000"
+        hb=0; while [ 1 ]; do let hb=$hb+1; devmem 0x1bfff0000 32 $hb; done &
+      '';
     };
 
     loader = {