add scratchpad config (default size = 4MB) | other misc comments (#383)

generators/example/src/main/scala/TopCakes.scala

@@ -0,0 +1,27 @@
+package example
+
+import chisel3._
+
+import freechips.rocketchip.subsystem.BaseSubsystem
+import freechips.rocketchip.config.{Field}
+import freechips.rocketchip.diplomacy.{LazyModule, AddressSet}
+import freechips.rocketchip.tilelink.{TLRAM}
+
+case class BackingScratchpadParams(
+  base: BigInt,
+  mask: BigInt)
+
+case object BackingScratchpadKey extends Field[Option[BackingScratchpadParams]](None)
+
+/**
+ * Trait to add a scratchpad on the mbus
+ */
+trait CanHaveBackingScratchpad { this: BaseSubsystem =>
+  private val portName = "Backing-Scratchpad"
+
+  val spadOpt = p(BackingScratchpadKey).map { param =>
+    val spad = LazyModule(new TLRAM(address=AddressSet(param.base, param.mask), beatBytes=mbus.beatBytes))
+    mbus.toVariableWidthSlave(Some(portName)) { spad.node }
+    spad
+  }
+}