sim: Refactor and simplify the drain API

The drain() call currently passes around a DrainManager pointer, which
is now completely pointless since there is only ever one global
DrainManager in the system. It also contains vestiges from the time
when SimObjects had to keep track of their child objects that needed
draining.

This changeset moves all of the DrainState handling to the Drainable
base class and changes the drain() and drainResume() calls to reflect
this. Particularly, the drain() call has been updated to take no
parameters (the DrainManager argument isn't needed) and return a
DrainState instead of an unsigned integer (there is no point returning
anything other than 0 or 1 any more). Drainable objects should return
either DrainState::Draining (equivalent to returning 1 in the old
system) if they need more time to drain or DrainState::Drained
(equivalent to returning 0 in the old system) if they are already in a
consistent state. Returning DrainState::Running is considered an
error.

Drain done signalling is now done through the signalDrainDone() method
in the Drainable class instead of using the DrainManager directly. The
new call checks if the state of the object is DrainState::Draining
before notifying the drain manager. This means that it is safe to call
signalDrainDone() without first checking if the simulator has
requested draining. The intention here is to reduce the code needed to
implement draining in simple objects.

4 files changed, 18 insertions, 31 deletions

diff --git a/src/cpu/minor/cpu.cc b/src/cpu/minor/cpu.cc
index ac1a18bf9..a93d0037d 100644
--- a/src/cpu/minor/cpu.cc
+++ b/src/cpu/minor/cpu.cc
@@ -47,8 +47,7 @@
 #include "debug/Quiesce.hh"
 
 MinorCPU::MinorCPU(MinorCPUParams *params) :
-    BaseCPU(params),
-    drainManager(NULL)
+    BaseCPU(params)
 {
     /* This is only written for one thread at the moment */
     Minor::MinorThread *thread;
@@ -194,39 +193,33 @@ MinorCPU::startup()
     activateContext(0);
 }
 
-unsigned int
-MinorCPU::drain(DrainManager *drain_manager)
+DrainState
+MinorCPU::drain()
 {
     DPRINTF(Drain, "MinorCPU drain\n");
 
-    drainManager = drain_manager;
-
     /* Need to suspend all threads and wait for Execute to idle.
      * Tell Fetch1 not to fetch */
-    unsigned int ret = pipeline->drain(drain_manager);
-
-    if (ret == 0)
+    if (pipeline->drain()) {
         DPRINTF(Drain, "MinorCPU drained\n");
-    else
+        return DrainState::Drained;
+    } else {
         DPRINTF(Drain, "MinorCPU not finished draining\n");
-
-    return ret;
+        return DrainState::Draining;
+    }
 }
 
 void
 MinorCPU::signalDrainDone()
 {
     DPRINTF(Drain, "MinorCPU drain done\n");
-    setDrainState(DrainState::Drained);
-    drainManager->signalDrainDone();
-    drainManager = NULL;
+    signalDrainDone();
 }
 
 void
 MinorCPU::drainResume()
 {
-    assert(getDrainState() == DrainState::Drained ||
-        getDrainState() == DrainState::Running);
+    assert(drainState() == DrainState::Drained);
 
     if (switchedOut()) {
         DPRINTF(Drain, "drainResume while switched out.  Ignoring\n");
@@ -242,8 +235,6 @@ MinorCPU::drainResume()
 
     wakeup();
     pipeline->drainResume();
-
-    setDrainState(DrainState::Running);
 }
 
 void
diff --git a/src/cpu/minor/cpu.hh b/src/cpu/minor/cpu.hh
index fba54b515..2e877d786 100644
--- a/src/cpu/minor/cpu.hh
+++ b/src/cpu/minor/cpu.hh
@@ -112,10 +112,6 @@ class MinorCPU : public BaseCPU
         virtual void recvTimingSnoopReq(PacketPtr pkt) { }
     };
 
-    /** The DrainManager passed into drain that needs be signalled when
-     *  draining is complete */
-    DrainManager *drainManager;
-
   protected:
      /** Return a reference to the data port. */
     MasterPort &getDataPort();
@@ -155,10 +151,10 @@ class MinorCPU : public BaseCPU
     void unserialize(CheckpointIn &cp);
 
     /** Drain interface */
-    unsigned int drain(DrainManager *drain_manager);
-    void drainResume();
-    /** Signal from Pipeline that MinorCPU should signal the DrainManager
-     *  that a drain is complete and set its drainState */
+    DrainState drain() M5_ATTR_OVERRIDE;
+    void drainResume() M5_ATTR_OVERRIDE;
+    /** Signal from Pipeline that MinorCPU should signal that a drain
+     *  is complete and set its drainState */
     void signalDrainDone();
     void memWriteback();
 
diff --git a/src/cpu/minor/pipeline.cc b/src/cpu/minor/pipeline.cc
index 9d802234b..39b7f31f9 100644
--- a/src/cpu/minor/pipeline.cc
+++ b/src/cpu/minor/pipeline.cc
@@ -192,8 +192,8 @@ Pipeline::wakeupFetch()
     execute.wakeupFetch();
 }
 
-unsigned int
-Pipeline::drain(DrainManager *manager)
+bool
+Pipeline::drain()
 {
     DPRINTF(MinorCPU, "Draining pipeline by halting inst fetches. "
         " Execution should drain naturally\n");
@@ -205,7 +205,7 @@ Pipeline::drain(DrainManager *manager)
     bool drained = isDrained();
     needToSignalDrained = !drained;
 
-    return (drained ? 0 : 1);
+    return drained;
 }
 
 void
diff --git a/src/cpu/minor/pipeline.hh b/src/cpu/minor/pipeline.hh
index 355a3c6c2..bf2071b02 100644
--- a/src/cpu/minor/pipeline.hh
+++ b/src/cpu/minor/pipeline.hh
@@ -115,7 +115,7 @@ class Pipeline : public Ticked
     void wakeupFetch();
 
     /** Try to drain the CPU */
-    unsigned int drain(DrainManager *manager);
+    bool drain();
 
     void drainResume();