Conflict with a ground net

[gabrielrodcanal]

Hi,
I have generated a simple benchmark to test my router, which is based on the custom Python router provided in the contest. I'm working on fixing overlaps, since this is not included in the base version, and I've found that I get collisions with GND signals. How is this possible? Aren't GND and VCC nets removed from the routing graph in the beginning of the program? In particular, Vivado complains there's a collision between one of my generated nets and GLOBAL_LOGIC0. When I inspect the conflicts with show_objects -name find_1 [get_nets -hierarchical -top_net_of_hierarchical_group -filter { ROUTE_STATUS == "CONFLICTS" } ] I get the following:

Any guidance here would be appreciated.

[eddieh-xlnx]

Hi @gabrielrodcanal. Thanks for posting!

I've found that I get collisions with GND signals. How is this possible? Aren't GND and VCC nets removed from the routing graph in the beginning of the program?

For nxroute-poc, no effort is made to remove or block all the routing resources used by the pre-routed GND/VCC/clock nets. There is, however, this block of code:

fpga24_routing_contest/networkx-proof-of-concept-router/nxroute-poc.py

Lines 384 to 391 in e8e0ee8

	# This net either has no stubs (fully routed) or it is a gnd/vcc net
	# Remove all its site pin nodes from the routing graph so that they can't
	# be used by other nets (otherwise Vivado reports a site pin conflict)
	for sp in self.extractSitePins(net.sources) + sinkPins:
	siteName,sinkName = s[sp.site],s[sp.pin]
	blockedNode = self.G.getNodeFromSitePin(siteName, sinkName)
	if blockedNode is not None:
	self.G.remove_node(blockedNode)

that is responsible for blocking the routing resources of just the site pin nodes (but not the actual routing nodes that connects site pins together) on those pre-routed nets. The reason for this is that Vivado has (at least) two types of conflicts: site pin conflicts and route conflicts. Site pin conflicts means that a net is overlapping with a site pin from another net -- and it's obvious who should "win" that resource, since the site pin is the only way for the latter net to reach the fabric. For a route conflict, it's less obvious (from Vivado's point of view) who really deserves that resource the most. However, from the point of view of this contest, static/pre-routed nets must be preserved and thus your router will need to avoid those nodes.

To keep things simple for this MVP nxroute, I prevent site pin conflicts from ever happening (code above) and so what you'll only ever see are route conflicts.

Long story short, you will need to remove the routing resources used by pre-routed nets from the graph yourself after which those routing conflicts should go away.

[gabrielrodcanal]

Hi @eddieh-xlnx. Thank you for your answer.

I guess it would be enough with removing just the sources and the sinks of the pre-routed nets instead of all their nodes in the graph?

[eddieh-xlnx]

I guess it would be enough with removing just the sources and the sinks of the pre-routed nets instead of all their nodes in the graph?

No, what I'm saying is that I currently remove the source/sink nodes of pre-routed nets from the graph. As you've found, that's not enough because to-be-routed nets can cause "route conflicts" with pre-routed nets. You'll have to go ahead and remove all pre-routed net nodes from the graph too.

[eddieh-xlnx]

You'll have to go ahead and remove all pre-routed net nodes from the graph too.

Resolved in #55