cpu: Make automatic transition to OFF optional

Add the power_gating_on_idle option to control whether a core
automatically enters the power gated state. The default behaviour is
to transition to clock gated when idle, but not to power gated. When
this option is set to true, the core automatically transitions to the
power gated state after a configurable latency.

Change-Id: Ida98c7fc532de4140d0e511c25613769b47b3702
Reviewed-on: https://gem5-review.googlesource.com/5741
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/cpu/BaseCPU.py

@@ -138,6 +138,10 @@ class BaseCPU(MemObject):
     pwr_gating_latency = Param.Cycles(300,
         "Latency to enter power gating state when all contexts are suspended")
 
+    power_gating_on_idle = Param.Bool(False, "Control whether the core goes "\
+        "to the OFF power state after all thread are disabled for "\
+        "pwr_gating_latency cycles")
+
     function_trace = Param.Bool(False, "Enable function trace")
     function_trace_start = Param.Tick(0, "Tick to start function trace")
 

src/cpu/base.cc

@@ -138,6 +138,7 @@ BaseCPU::BaseCPU(Params *p, bool is_checker)
       addressMonitor(p->numThreads),
       syscallRetryLatency(p->syscallRetryLatency),
       pwrGatingLatency(p->pwr_gating_latency),
+      powerGatingOnIdle(p->power_gating_on_idle),
       enterPwrGatingEvent([this]{ enterPwrGating(); }, name())
 {
     // if Python did not provide a valid ID, do it here
@@ -493,7 +494,8 @@ BaseCPU::schedulePowerGatingEvent()
             return;
     }
 
-    if (ClockedObject::pwrState() == Enums::PwrState::CLK_GATED) {
+    if (ClockedObject::pwrState() == Enums::PwrState::CLK_GATED &&
+        powerGatingOnIdle) {
         assert(!enterPwrGatingEvent.scheduled());
         // Schedule a power gating event when clock gated for the specified
         // amount of time
@@ -536,8 +538,12 @@ BaseCPU::suspendContext(ThreadID thread_num)
     // All CPU threads suspended, enter lower power state for the CPU
     ClockedObject::pwrState(Enums::PwrState::CLK_GATED);
 
-    //Schedule power gating event when clock gated for a configurable cycles
-    schedule(enterPwrGatingEvent, clockEdge(pwrGatingLatency));
+    // If pwrGatingLatency is set to 0 then this mechanism is disabled
+    if (powerGatingOnIdle) {
+        // Schedule power gating event when clock gated for pwrGatingLatency
+        // cycles
+        schedule(enterPwrGatingEvent, clockEdge(pwrGatingLatency));
+    }
 }
 
 void

src/cpu/base.hh

@@ -588,10 +588,13 @@ class BaseCPU : public MemObject
     bool waitForRemoteGDB() const;
 
     Cycles syscallRetryLatency;
+
   // Enables CPU to enter power gating on a configurable cycle count
   protected:
-    const Cycles pwrGatingLatency;
     void enterPwrGating();
+
+    const Cycles pwrGatingLatency;
+    const bool powerGatingOnIdle;
     EventFunctionWrapper enterPwrGatingEvent;
 };