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.

3 files changed, 20 insertions, 46 deletions

diff --git a/src/cpu/o3/fetch_impl.hh b/src/cpu/o3/fetch_impl.hh
index 81d70bd61..caafa3fe3 100644
--- a/src/cpu/o3/fetch_impl.hh
+++ b/src/cpu/o3/fetch_impl.hh
@@ -363,8 +363,6 @@ DefaultFetch<Impl>::processCacheCompletion(PacketPtr pkt)
 
     DPRINTF(Fetch, "[tid:%u] Waking up from cache miss.\n", tid);
 
-    assert(!pkt->wasNacked());
-
     // Only change the status if it's still waiting on the icache access
     // to return.
     if (fetchStatus[tid] != IcacheWaitResponse ||
diff --git a/src/cpu/o3/lsq_unit_impl.hh b/src/cpu/o3/lsq_unit_impl.hh
index a878b1540..7c98b99fb 100644
--- a/src/cpu/o3/lsq_unit_impl.hh
+++ b/src/cpu/o3/lsq_unit_impl.hh
@@ -95,8 +95,6 @@ LSQUnit<Impl>::completeDataAccess(PacketPtr pkt)
 
     //iewStage->ldstQueue.removeMSHR(inst->threadNumber,inst->seqNum);
 
-    assert(!pkt->wasNacked());
-
     // If this is a split access, wait until all packets are received.
     if (TheISA::HasUnalignedMemAcc && !state->complete()) {
         delete pkt->req;
diff --git a/src/cpu/simple/timing.cc b/src/cpu/simple/timing.cc
index 6a9fe7efc..9022845ce 100644
--- a/src/cpu/simple/timing.cc
+++ b/src/cpu/simple/timing.cc
@@ -719,25 +719,14 @@ TimingSimpleCPU::IcachePort::ITickEvent::process()
 bool
 TimingSimpleCPU::IcachePort::recvTimingResp(PacketPtr pkt)
 {
-    if (!pkt->wasNacked()) {
-        DPRINTF(SimpleCPU, "Received timing response %#x\n", pkt->getAddr());
-        // delay processing of returned data until next CPU clock edge
-        Tick next_tick = cpu->nextCycle(curTick());
-
-        if (next_tick == curTick())
-            cpu->completeIfetch(pkt);
-        else
-            tickEvent.schedule(pkt, next_tick);
+    DPRINTF(SimpleCPU, "Received timing response %#x\n", pkt->getAddr());
+    // delay processing of returned data until next CPU clock edge
+    Tick next_tick = cpu->nextCycle();
 
-        return true;
-    } else {
-        assert(cpu->_status == IcacheWaitResponse);
-        pkt->reinitNacked();
-        if (!sendTimingReq(pkt)) {
-            cpu->_status = IcacheRetry;
-            cpu->ifetch_pkt = pkt;
-        }
-    }
+    if (next_tick == curTick())
+        cpu->completeIfetch(pkt);
+    else
+        tickEvent.schedule(pkt, next_tick);
 
     return true;
 }
@@ -839,32 +828,21 @@ TimingSimpleCPU::completeDrain()
 bool
 TimingSimpleCPU::DcachePort::recvTimingResp(PacketPtr pkt)
 {
-    if (!pkt->wasNacked()) {
-        // delay processing of returned data until next CPU clock edge
-        Tick next_tick = cpu->nextCycle(curTick());
+    // delay processing of returned data until next CPU clock edge
+    Tick next_tick = cpu->nextCycle();
 
-        if (next_tick == curTick()) {
-            cpu->completeDataAccess(pkt);
+    if (next_tick == curTick()) {
+        cpu->completeDataAccess(pkt);
+    } else {
+        if (!tickEvent.scheduled()) {
+            tickEvent.schedule(pkt, next_tick);
         } else {
-            if (!tickEvent.scheduled()) {
-                tickEvent.schedule(pkt, next_tick);
-            } else {
-                // In the case of a split transaction and a cache that is
-                // faster than a CPU we could get two responses before
-                // next_tick expires
-                if (!retryEvent.scheduled())
-                    cpu->schedule(retryEvent, next_tick);
-                return false;
-            }
-        }
-
-        return true;
-    } else  {
-        assert(cpu->_status == DcacheWaitResponse);
-        pkt->reinitNacked();
-        if (!sendTimingReq(pkt)) {
-            cpu->_status = DcacheRetry;
-            cpu->dcache_pkt = pkt;
+            // In the case of a split transaction and a cache that is
+            // faster than a CPU we could get two responses before
+            // next_tick expires
+            if (!retryEvent.scheduled())
+                cpu->schedule(retryEvent, next_tick);
+            return false;
         }
     }