xref: /gem5/src/mem/XBar.py

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# Authors: Nathan Binkert
#          Andreas Hansson

from MemObject import MemObject
from System import System
from m5.params import *
from m5.proxy import *
from m5.SimObject import SimObject

class BaseXBar(MemObject):
    type = 'BaseXBar'
    abstract = True
    cxx_header = "mem/xbar.hh"

    slave = VectorSlavePort("Vector port for connecting masters")
    master = VectorMasterPort("Vector port for connecting slaves")

    # Latencies governing the time taken for the variuos paths a
    # packet has through the crossbar. Note that the crossbar itself
    # does not add the latency due to assumptions in the coherency
    # mechanism. Instead the latency is annotated on the packet and
    # left to the neighbouring modules.
    #
    # A request incurs the frontend latency, possibly snoop filter
    # lookup latency, and forward latency. A response incurs the
    # response latency. Frontend latency encompasses arbitration and
    # deciding what to do when a request arrives. the forward latency
    # 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")

    # Width governing the throughput of the crossbar
    width = Param.Unsigned(8, "Datapath width per port (bytes)")

    # The default port can be left unconnected, or be used to connect
    # a default slave port
    default = MasterPort("Port for connecting an optional default slave")

    # The default port can be used unconditionally, or based on
    # address range, in which case it may overlap with other
    # ports. The default range is always checked first, thus creating
    # a two-level hierarchical lookup. This is useful e.g. for the PCI
    # xbar configuration.
    use_default_range = Param.Bool(False, "Perform address mapping for " \
                                       "the default port")

class NoncoherentXBar(BaseXBar):
    type = 'NoncoherentXBar'
    cxx_header = "mem/noncoherent_xbar.hh"

class CoherentXBar(BaseXBar):
    type = 'CoherentXBar'
    cxx_header = "mem/coherent_xbar.hh"

    # The coherent crossbar additionally has snoop responses that are
    # forwarded after a specific latency.
    snoop_response_latency = Param.Cycles(4, "Snoop response latency")

    # An optional snoop filter
    snoop_filter = Param.SnoopFilter(NULL, "Selected snoop filter")

    system = Param.System(Parent.any, "System that the crossbar belongs to.")

class SnoopFilter(SimObject):
    type = 'SnoopFilter'
    cxx_header = "mem/snoop_filter.hh"

    # Lookup latency of the snoop filter, added to requests that pass
    # through a coherent crossbar.
    lookup_latency = Param.Cycles(1, "Lookup latency")

    system = Param.System(Parent.any, "System that the crossbar belongs to.")