Allow reset vector to be set dynamically

A chip's power-up sequence, or awake-from-sleep sequence, may wish to
set the reset PC based upon dynamic properties, e.g., the settings of
external pins.  Support this by passing the reset vector to the Coreplex.
ExampleTop simply hard-wires the reset vector, as was the case before.

Additionally, allow MTVEC to *not* be reset.  In most cases, including
riscv-tests, pk, and bbl, overriding MTVEC is one of the first things
that the boot sequence does.  So the reset value is superfluous.

src/main/scala/rocket/csr.scala

@@ -227,7 +227,11 @@ class CSRFile(implicit p: Parameters) extends CoreModule()(p)
   val reg_mcause = Reg(Bits(width = xLen))
   val reg_mbadaddr = Reg(UInt(width = vaddrBitsExtended))
   val reg_mscratch = Reg(Bits(width = xLen))
-  val reg_mtvec = Reg(init=UInt(p(MtvecInit), paddrBits min xLen))
+  val mtvecWidth = paddrBits min xLen
+  val reg_mtvec = p(MtvecInit) match {
+    case Some(addr) => Reg(init=UInt(addr, mtvecWidth))
+    case None => Reg(UInt(width = mtvecWidth))
+  }
   val reg_mucounteren = Reg(UInt(width = 32))
   val reg_mscounteren = Reg(UInt(width = 32))
   val delegable_counters = (BigInt(1) << (nPerfCounters + CSR.firstHPM)) - 1

src/main/scala/rocket/frontend.scala

@@ -35,6 +35,7 @@ class Frontend(implicit p: Parameters) extends CoreModule()(p) with HasL1CachePa
     val cpu = new FrontendIO().flip
     val ptw = new TLBPTWIO()
     val mem = new ClientUncachedTileLinkIO
+    val resetVector = UInt(INPUT, vaddrBitsExtended)
   }
 
   val icache = Module(new ICache(latency = 2))
@@ -45,7 +46,7 @@ class Frontend(implicit p: Parameters) extends CoreModule()(p) with HasL1CachePa
   val s1_speculative = Reg(Bool())
   val s1_same_block = Reg(Bool())
   val s2_valid = Reg(init=Bool(true))
-  val s2_pc = Reg(init=UInt(p(ResetVector)))
+  val s2_pc = Reg(init=io.resetVector)
   val s2_btb_resp_valid = Reg(init=Bool(false))
   val s2_btb_resp_bits = Reg(new BTBResp)
   val s2_xcpt_if = Reg(init=Bool(false))

src/main/scala/rocket/rocket.scala

@@ -27,8 +27,7 @@ case object FastJAL extends Field[Boolean]
 case object CoreInstBits extends Field[Int]
 case object NCustomMRWCSRs extends Field[Int]
 case object MtvecWritable extends Field[Boolean]
-case object MtvecInit extends Field[BigInt]
-case object ResetVector extends Field[BigInt]
+case object MtvecInit extends Field[Option[BigInt]]
 case object NBreakpoints extends Field[Int]
 case object NPerfCounters extends Field[Int]
 case object NPerfEvents extends Field[Int]

src/main/scala/rocket/tile.scala

@@ -34,6 +34,7 @@ abstract class Tile(clockSignal: Clock = null, resetSignal: Bool = null)
     val hartid = UInt(INPUT, p(XLen))
     val interrupts = new TileInterrupts().asInput
     val slave = (p(DataScratchpadSize) > 0).option(new ClientUncachedTileLinkIO().flip)
+    val resetVector = UInt(INPUT, p(XLen))
   }
 
   val io = new TileIO
@@ -58,6 +59,7 @@ class RocketTile(clockSignal: Clock = null, resetSignal: Bool = null)
   core.io.interrupts := io.interrupts
   core.io.hartid := io.hartid
   icache.io.cpu <> core.io.imem
+  icache.io.resetVector := io.resetVector
 
   val fpuOpt = p(FPUKey).map(cfg => Module(new FPU(cfg)))
   fpuOpt.foreach(fpu => core.io.fpu <> fpu.io)