Packet: Remove NACKs from packet and its use in endpoints

This patch removes the NACK frrom the packet as there is no longer any
module in the system that issues them (the bridge was the only one and
the previous patch removes that).

The handling of NACKs was mostly avoided throughout the code base, by
using e.g. panic or assert false, but in a few locations the NACKs
were actually dealt with (although NACKs never occured in any of the
regressions). Most notably, the DMA port will now never receive a NACK
and the backoff time is thus never changed. As a consequence, the
entire backoff mechanism (similar to a PCI bus) is now removed and the
DMA port entirely relies on the bus performing the arbitration and
issuing a retry when appropriate. This is more in line with e.g. PCIe.

Surprisingly, this patch has no impact on any of the regressions. As
mentioned in the patch that removes the NACK from the bridge, a
follow-up patch should change the request and response buffer size for
at least one regression to also verify that the system behaves as
expected when the bridge fills up.

4 files changed, 13 insertions, 63 deletions

diff --git a/src/dev/Device.py b/src/dev/Device.py
index 60c21df91..b1a4f69bc 100644
--- a/src/dev/Device.py
+++ b/src/dev/Device.py
@@ -46,11 +46,6 @@ class DmaDevice(PioDevice):
     type = 'DmaDevice'
     abstract = True
     dma = MasterPort("DMA port")
-    min_backoff_delay = Param.Latency('4ns',
-      "min time between a nack packet being received and the next request made by the device")
-    max_backoff_delay = Param.Latency('10us',
-      "max time between a nack packet being received and the next request made by the device")
-
 
 
 class IsaFake(BasicPioDevice):
diff --git a/src/dev/copy_engine.cc b/src/dev/copy_engine.cc
index bb15abab6..77cc735a9 100644
--- a/src/dev/copy_engine.cc
+++ b/src/dev/copy_engine.cc
@@ -78,8 +78,7 @@ CopyEngine::CopyEngine(const Params *p)
 
 
 CopyEngine::CopyEngineChannel::CopyEngineChannel(CopyEngine *_ce, int cid)
-    : cePort(_ce, _ce->sys, _ce->params()->min_backoff_delay,
-             _ce->params()->max_backoff_delay),
+    : cePort(_ce, _ce->sys),
       ce(_ce), channelId(cid), busy(false), underReset(false),
     refreshNext(false), latBeforeBegin(ce->params()->latBeforeBegin),
     latAfterCompletion(ce->params()->latAfterCompletion),
diff --git a/src/dev/dma_device.cc b/src/dev/dma_device.cc
index 5a2f52df1..80253129f 100644
--- a/src/dev/dma_device.cc
+++ b/src/dev/dma_device.cc
@@ -47,36 +47,18 @@
 #include "dev/dma_device.hh"
 #include "sim/system.hh"
 
-DmaPort::DmaPort(MemObject *dev, System *s, Tick min_backoff, Tick max_backoff)
+DmaPort::DmaPort(MemObject *dev, System *s)
     : MasterPort(dev->name() + ".dma", dev), device(dev), sys(s),
       masterId(s->getMasterId(dev->name())),
       pendingCount(0), drainEvent(NULL),
-      backoffTime(0), minBackoffDelay(min_backoff),
-      maxBackoffDelay(max_backoff), inRetry(false),
-      backoffEvent(this)
+      inRetry(false)
 { }
 
 bool
 DmaPort::recvTimingResp(PacketPtr pkt)
 {
-    if (pkt->wasNacked()) {
-        DPRINTF(DMA, "Received nacked %s addr %#x\n",
-                pkt->cmdString(), pkt->getAddr());
-
-        if (backoffTime < minBackoffDelay)
-            backoffTime = minBackoffDelay;
-        else if (backoffTime < maxBackoffDelay)
-            backoffTime <<= 1;
-
-        device->reschedule(backoffEvent, curTick() + backoffTime, true);
-
-        DPRINTF(DMA, "Backoff time set to %d ticks\n", backoffTime);
-
-        pkt->reinitNacked();
-        queueDma(pkt, true);
-    } else if (pkt->senderState) {
+    if (pkt->senderState) {
         DmaReqState *state;
-        backoffTime >>= 2;
 
         DPRINTF(DMA, "Received response %s addr %#x size %#x\n",
                 pkt->cmdString(), pkt->getAddr(), pkt->req->getSize());
@@ -116,8 +98,7 @@ DmaPort::recvTimingResp(PacketPtr pkt)
 }
 
 DmaDevice::DmaDevice(const Params *p)
-    : PioDevice(p), dmaPort(this, sys, params()->min_backoff_delay,
-                            params()->max_backoff_delay)
+    : PioDevice(p), dmaPort(this, sys)
 { }
 
 void
@@ -168,16 +149,10 @@ DmaPort::recvRetry()
             inRetry = true;
             DPRINTF(DMA, "-- Failed, queued\n");
         }
-    } while (!backoffTime &&  result && transmitList.size());
+    } while (result && transmitList.size());
 
-    if (transmitList.size() && backoffTime && !inRetry) {
-        DPRINTF(DMA, "Scheduling backoff for %d\n", curTick()+backoffTime);
-        if (!backoffEvent.scheduled())
-            device->schedule(backoffEvent, backoffTime + curTick());
-    }
-    DPRINTF(DMA, "TransmitList: %d, backoffTime: %d inRetry: %d es: %d\n",
-            transmitList.size(), backoffTime, inRetry,
-            backoffEvent.scheduled());
+    DPRINTF(DMA, "TransmitList: %d, inRetry: %d\n",
+            transmitList.size(), inRetry);
 }
 
 void
@@ -231,8 +206,8 @@ DmaPort::sendDma()
 
     Enums::MemoryMode state = sys->getMemoryMode();
     if (state == Enums::timing) {
-        if (backoffEvent.scheduled() || inRetry) {
-            DPRINTF(DMA, "Can't send immediately, waiting for retry or backoff timer\n");
+        if (inRetry) {
+            DPRINTF(DMA, "Can't send immediately, waiting for retry\n");
             return;
         }
 
@@ -249,14 +224,7 @@ DmaPort::sendDma()
                 inRetry = true;
                 DPRINTF(DMA, "-- Failed: queued\n");
             }
-        } while (result && !backoffTime && transmitList.size());
-
-        if (transmitList.size() && backoffTime && !inRetry &&
-                !backoffEvent.scheduled()) {
-            DPRINTF(DMA, "-- Scheduling backoff timer for %d\n",
-                    backoffTime+curTick());
-            device->schedule(backoffEvent, backoffTime + curTick());
-        }
+        } while (result && transmitList.size());
     } else if (state == Enums::atomic) {
         transmitList.pop_front();
 
diff --git a/src/dev/dma_device.hh b/src/dev/dma_device.hh
index ccf388fa4..691a21749 100644
--- a/src/dev/dma_device.hh
+++ b/src/dev/dma_device.hh
@@ -87,16 +87,6 @@ class DmaPort : public MasterPort
      * here.*/
     Event *drainEvent;
 
-    /** time to wait between sending another packet, increases as NACKs are
-     * recived, decreases as responses are recived. */
-    Tick backoffTime;
-
-    /** Minimum time that device should back off for after failed sendTiming */
-    Tick minBackoffDelay;
-
-    /** Maximum time that device should back off for after failed sendTiming */
-    Tick maxBackoffDelay;
-
     /** If the port is currently waiting for a retry before it can send whatever
      * it is that it's sending. */
     bool inRetry;
@@ -108,11 +98,9 @@ class DmaPort : public MasterPort
     void queueDma(PacketPtr pkt, bool front = false);
     void sendDma();
 
-    /** event to give us a kick every time we backoff time is reached. */
-    EventWrapper<DmaPort, &DmaPort::sendDma> backoffEvent;
-
   public:
-    DmaPort(MemObject *dev, System *s, Tick min_backoff, Tick max_backoff);
+
+    DmaPort(MemObject *dev, System *s);
 
     void dmaAction(Packet::Command cmd, Addr addr, int size, Event *event,
                    uint8_t *data, Tick delay, Request::Flags flag = 0);