# is the latency involved once a decision is made to forward the
# request. The response latency, is similar to the forward
# latency, but for responses rather than requests.
- frontend_latency = Param.Cycles(3, "Frontend latency")
- forward_latency = Param.Cycles(4, "Forward latency")
- response_latency = Param.Cycles(2, "Response latency")
+ frontend_latency = Param.Cycles("Frontend latency")
+ forward_latency = Param.Cycles("Forward latency")
+ response_latency = Param.Cycles("Response latency")
# Width governing the throughput of the crossbar
- width = Param.Unsigned(8, "Datapath width per port (bytes)")
+ width = Param.Unsigned("Datapath width per port (bytes)")
# The default port can be left unconnected, or be used to connect
# a default slave port
# The coherent crossbar additionally has snoop responses that are
# forwarded after a specific latency.
- snoop_response_latency = Param.Cycles(4, "Snoop response latency")
+ snoop_response_latency = Param.Cycles("Snoop response latency")
# An optional snoop filter
snoop_filter = Param.SnoopFilter(NULL, "Selected snoop filter")
lookup_latency = Param.Cycles(1, "Lookup latency")
system = Param.System(Parent.any, "System that the crossbar belongs to.")
+
+# We use a coherent crossbar to connect multiple masters to the L2
+# caches. Normally this crossbar would be part of the cache itself.
+class L2XBar(CoherentXBar):
+ # 256-bit crossbar by default
+ width = 32
+
+ # Assume that most of this is covered by the cache latencies, with
+ # no more than a single pipeline stage for any packet.
+ frontend_latency = 1
+ forward_latency = 0
+ response_latency = 1
+ snoop_response_latency = 1
+
+# One of the key coherent crossbar instances is the system
+# interconnect, tying together the CPU clusters, GPUs, and any I/O
+# coherent masters, and DRAM controllers.
+class SystemXBar(CoherentXBar):
+ # 128-bit crossbar by default
+ width = 16
+
+ # A handful pipeline stages for each portion of the latency
+ # contributions.
+ frontend_latency = 3
+ forward_latency = 4
+ response_latency = 2
+ snoop_response_latency = 4
+
+# In addition to the system interconnect, we typically also have one
+# or more on-chip I/O crossbars. Note that at some point we might want
+# to also define an off-chip I/O crossbar such as PCIe.
+class IOXBar(NoncoherentXBar):
+ # 128-bit crossbar by default
+ width = 16
+
+ # Assume a simpler datapath than a coherent crossbar, incuring
+ # less pipeline stages for decision making and forwarding of
+ # requests.
+ frontend_latency = 2
+ forward_latency = 1
+ response_latency = 2