From 00481d1f192c832d654379c2296d5b6020c12b1a Mon Sep 17 00:00:00 2001 From: Gabe Black Date: Sun, 8 Oct 2006 14:08:58 -0400 Subject: [PATCH] A possible implementation of a multiplexed bus. --HG-- extra : convert_revision : 3c560eda12ffd8ca539c91024baf2770b963ede8 --- src/mem/bus.cc | 164 +++++++++++++++++++++++++++++++------------------ src/mem/bus.hh | 53 ++++++++++++++-- 2 files changed, 152 insertions(+), 65 deletions(-) diff --git a/src/mem/bus.cc b/src/mem/bus.cc index e3b395afc..fff3dfed6 100644 --- a/src/mem/bus.cc +++ b/src/mem/bus.cc @@ -67,6 +67,44 @@ Bus::init() (*intIter)->sendStatusChange(Port::RangeChange); } +Bus::BusFreeEvent::BusFreeEvent(Bus *_bus) : Event(&mainEventQueue), bus(_bus) +{} + +void Bus::BusFreeEvent::process() +{ + bus->recvRetry(0); +} + +const char * Bus::BusFreeEvent::description() +{ + return "bus became available"; +} + +void +Bus::occupyBus(int numCycles) +{ + //Move up when the bus will next be free + //We avoid the use of divide by adding repeatedly + //This should be faster if the value is updated frequently, but should + //be may be slower otherwise. + + //Bring tickNextIdle up to the present tick + //There is some potential ambiguity where a cycle starts, which might make + //a difference when devices are acting right around a cycle boundary. Using + //a < allows things which happen exactly on a cycle boundary to take up only + //the following cycle. Anthing that happens later will have to "wait" for the + //end of that cycle, and then start using the bus after that. + while (tickNextIdle < curTick) + tickNextIdle += clock; + //Advance it numCycles bus cycles. + //XXX Should this use the repeating add trick as well? + tickNextIdle += (numCycles * clock); + if (!busIdle.scheduled()) { + busIdle.schedule(tickNextIdle); + } else { + busIdle.reschedule(tickNextIdle); + } +} /** Function called by the port when the bus is receiving a Timing * transaction.*/ @@ -77,83 +115,89 @@ Bus::recvTiming(Packet *pkt) DPRINTF(Bus, "recvTiming: packet src %d dest %d addr 0x%x cmd %s\n", pkt->getSrc(), pkt->getDest(), pkt->getAddr(), pkt->cmdString()); - short dest = pkt->getDest(); - //if (pkt->isRequest() && curTick < tickAddrLastUsed || - // (pkt->isResponse() || pkt->hasData()) && curTick < tickDataLastUsed) { - //We're going to need resources that have already been committed - //Send this guy to the back of the line - //We don't need to worry about scheduling an event to deal with when the - //bus is freed because that's handled when tick*LastUsed is incremented. - // retryList.push_back(interfaces[pkt->getSrc()]); - // return false; - //} + Port *pktPort = interfaces[pkt->getSrc()]; - if (dest == Packet::Broadcast) { - if ( timingSnoopPhase1(pkt) ) - { - timingSnoopPhase2(pkt); - port = findPort(pkt->getAddr(), pkt->getSrc()); - } - else - { - //Snoop didn't succeed - retryList.push_back(interfaces[pkt->getSrc()]); - return false; - } - } else { - assert(dest >= 0 && dest < interfaces.size()); - assert(dest != pkt->getSrc()); // catch infinite loops - port = interfaces[dest]; + // If the bus is busy, or other devices are in line ahead of the current one, + // put this device on the retry list. + if (tickNextIdle > curTick || (retryList.size() && pktPort != retryingPort)) { + addToRetryList(pktPort); + return false; } + // If the bus is blocked, make the device wait. + if (!(port = findDestPort(pkt, pkt->getSrc()))) { + addToRetryList(pktPort); + return false; + } + + // The packet will be sent. Figure out how long it occupies the bus. + int numCycles = 0; + // Requests need one cycle to send an address + if (pkt->isRequest()) + numCycles++; + else if (pkt->isResponse() || pkt->hasData()) { + // If a packet has data, it needs ceil(size/width) cycles to send it + // We're using the "adding instead of dividing" trick again here + if (pkt->hasData()) { + int dataSize = pkt->getSize(); + for (int transmitted = 0; transmitted < dataSize; + transmitted += width) { + numCycles++; + } + } else { + // If the packet didn't have data, it must have been a response. + // Those use the bus for one cycle to send their data. + numCycles++; + } + } + + occupyBus(numCycles); if (port->sendTiming(pkt)) { - // Packet was successfully sent. - // Figure out what resources were used, and then return true. - //if (pkt->isRequest()) { - // The address bus will be used for one cycle - // while (tickAddrLastUsed <= curTick) - // tickAddrLastUsed += clock; - //} - //if (pkt->isResponse() || pkt->hasData()) { - // Use up the data bus for at least one bus cycle - // while (tickDataLastUsed <= curTick) - // tickDataLastUsed += clock; - // Use up the data bus for however many cycles remain - // if (pkt->hasData()) { - // int dataSize = pkt->getSize(); - // for (int transmitted = width; transmitted < dataSize; - // transmitted += width) { - // tickDataLastUsed += clock; - // } - // } - //} + // Packet was successfully sent. Return true. return true; } - // packet not successfully sent - retryList.push_back(interfaces[pkt->getSrc()]); + // Packet not successfully sent. Leave or put it on the retry list. + addToRetryList(pktPort); return false; } void Bus::recvRetry(int id) { - // Go through all the elements on the list calling sendRetry on each - // This is not very efficient at all but it works. Ultimately we should end - // up with something that is more intelligent. - int initialSize = retryList.size(); - int i; - Port *p; - - for (i = 0; i < initialSize; i++) { - assert(retryList.size() > 0); - p = retryList.front(); - retryList.pop_front(); - p->sendRetry(); + //If there's anything waiting... + if (retryList.size()) { + retryingPort = retryList.front(); + retryingPort->sendRetry(); + //If the retryingPort pointer isn't null, either sendTiming wasn't + //called, or it was and the packet was successfully sent. + if (retryingPort) { + retryList.pop_front(); + retryingPort = 0; + } } } +Port * +Bus::findDestPort(PacketPtr pkt, int id) +{ + Port * port = NULL; + short dest = pkt->getDest(); + + if (dest == Packet::Broadcast) { + if (timingSnoopPhase1(pkt)) { + timingSnoopPhase2(pkt); + port = findPort(pkt->getAddr(), pkt->getSrc()); + } + //else, port stays NULL + } else { + assert(dest >= 0 && dest < interfaces.size()); + assert(dest != pkt->getSrc()); // catch infinite loops + port = interfaces[dest]; + } + return port; +} Port * Bus::findPort(Addr addr, int id) diff --git a/src/mem/bus.hh b/src/mem/bus.hh index 9dd666304..96f1152a6 100644 --- a/src/mem/bus.hh +++ b/src/mem/bus.hh @@ -46,6 +46,7 @@ #include "mem/packet.hh" #include "mem/port.hh" #include "mem/request.hh" +#include "sim/eventq.hh" class Bus : public MemObject { @@ -55,10 +56,8 @@ class Bus : public MemObject int clock; /** the width of the bus in bits */ int width; - /** the last tick the address bus was used */ - Tick tickAddrLastUsed; - /** the last tick the data bus was used */ - Tick tickDataLastUsed; + /** the next tick at which the bus will be idle */ + Tick tickNextIdle; static const int defaultId = -1; @@ -101,6 +100,15 @@ class Bus : public MemObject */ Port *findPort(Addr addr, int id); + /** Finds the port a packet should be sent to. If the bus is blocked, no port + * is returned. + * @param pkt Packet to find a destination port for. + * @param id Id of the port this packet was received from + * (to prevent loops) + */ + + Port *findDestPort(PacketPtr pkt, int id); + /** Find all ports with a matching snoop range, except src port. Keep in mind * that the ranges shouldn't overlap or you will get a double snoop to the same * interface.and the cache will assert out. @@ -189,6 +197,22 @@ class Bus : public MemObject }; + class BusFreeEvent : public Event + { + Bus * bus; + + public: + BusFreeEvent(Bus * _bus); + void process(); + const char *description(); + }; + + BusFreeEvent busIdle; + + void occupyBus(int numCycles); + + Port * retryingPort; + /** An array of pointers to the peer port interfaces connected to this bus.*/ std::vector interfaces; @@ -197,6 +221,23 @@ class Bus : public MemObject * original send failed for whatever reason.*/ std::list retryList; + void addToRetryList(Port * port) + { + if (!retryingPort) { + // The device wasn't retrying a packet, or wasn't at an appropriate + // time. + retryList.push_back(port); + } else { + // The device was retrying a packet. It didn't work, so we'll leave + // it at the head of the retry list. + retryingPort = 0; + + // We shouldn't be receiving a packet from one port when a different + // one is retrying. + assert(port == retryingPort); + } + } + /** Port that handles requests that don't match any of the interfaces.*/ Port *defaultPort; @@ -209,9 +250,11 @@ class Bus : public MemObject Bus(const std::string &n, int bus_id, int _clock, int _width) : MemObject(n), busId(bus_id), clock(_clock), width(_width), - tickAddrLastUsed(0), tickDataLastUsed(0), defaultPort(NULL) + tickNextIdle(0), busIdle(this), retryingPort(0), defaultPort(NULL) { + //Both the width and clock period must be positive assert(width); + assert(clock); } };