Add chosen node to DTS to enable dynamic earlycon select

generators/chipyard/src/main/scala/Subsystem.scala

@@ -12,6 +12,7 @@ import freechips.rocketchip.prci._
 import freechips.rocketchip.config.{Field, Parameters}
 import freechips.rocketchip.devices.tilelink._
 import freechips.rocketchip.devices.debug.{HasPeripheryDebug, HasPeripheryDebugModuleImp, ExportDebug, DebugModuleKey}
+import sifive.blocks.devices.uart.{HasPeripheryUART, PeripheryUARTKey}
 import freechips.rocketchip.diplomacy._
 import freechips.rocketchip.tile._
 import freechips.rocketchip.tilelink._
@@ -45,9 +46,35 @@ trait CanHaveHTIF { this: BaseSubsystem =>
   }
 }
 
+// This trait adds the "chosen" node to DTS, which
+// can be used to pass information to OS about the earlycon
+case object ChosenInDTS extends Field[Boolean](true)
+trait CanHaveChosenInDTS { this: BaseSubsystem =>
+  if (p(ChosenInDTS)) {
+    println("got here")
+    this match {
+      case t: HasPeripheryUART if (!p(PeripheryUARTKey).isEmpty) => {
+        println("Add chosen resource")
+        val chosen = new Device {
+          def describe(resources: ResourceBindings): Description = {
+            val stdout = resources("stdout").map(_.value)
+            Description("chosen", resources("uart").headOption.map { case Binding(_, value) =>
+              "stdout-path" -> Seq(value)
+            }.toMap)
+          }
+        }
+        ResourceBinding {
+          t.uarts.foreach(u => Resource(chosen, "uart").bind(ResourceAlias(u.device.label)))
+        }
+      }
+    }
+  }
+}
+
 class ChipyardSubsystem(implicit p: Parameters) extends BaseSubsystem
   with HasTiles
   with CanHaveHTIF
+  with CanHaveChosenInDTS
 {
   def coreMonitorBundles = tiles.map {
     case r: RocketTile => r.module.core.rocketImpl.coreMonitorBundle