compiler: add support for debugging through JLinkGDBServer

Some J-Link targets aren't supported in OpenOCD (or would need some
special configuration), so also give the option to use JLinkGDBServer
instead.

compileopts/target.go

@@ -47,6 +47,7 @@ type TargetSpec struct {
 	OpenOCDInterface string   `json:"openocd-interface"`
 	OpenOCDTarget    string   `json:"openocd-target"`
 	OpenOCDTransport string   `json:"openocd-transport"`
+	JLinkDevice      string   `json:"jlink-device"`
 }
 
 // copyProperties copies all properties that are set in spec2 into itself.
@@ -122,6 +123,9 @@ func (spec *TargetSpec) copyProperties(spec2 *TargetSpec) {
 	if spec2.OpenOCDTransport != "" {
 		spec.OpenOCDTransport = spec2.OpenOCDTransport
 	}
+	if spec2.JLinkDevice != "" {
+		spec.JLinkDevice = spec2.JLinkDevice
+	}
 }
 
 // load reads a target specification from the JSON in the given io.Reader. It

main.go

@@ -288,12 +288,13 @@ func FlashGDB(pkgName string, ocdOutput bool, options *compileopts.Options) erro
 		gdbInterface, openocdInterface := config.Programmer()
 		switch gdbInterface {
 		case "msd", "command", "":
-			if openocdInterface != "" && config.Target.OpenOCDTarget != "" {
-				gdbInterface = "openocd"
-			}
 			if len(config.Target.Emulator) != 0 {
 				// Assume QEMU as an emulator.
 				gdbInterface = "qemu"
+			} else if openocdInterface != "" && config.Target.OpenOCDTarget != "" {
+				gdbInterface = "openocd"
+			} else if config.Target.JLinkDevice != "" {
+				gdbInterface = "jlink"
 			}
 		}
 
@@ -331,6 +332,31 @@ func FlashGDB(pkgName string, ocdOutput bool, options *compileopts.Options) erro
 				// Maybe we should send a .Kill() after x seconds?
 				daemon.Wait()
 			}()
+		case "jlink":
+			gdbCommands = append(gdbCommands, "target remote :2331", "load", "monitor reset halt")
+
+			// We need a separate debugging daemon for on-chip debugging.
+			daemon := exec.Command("JLinkGDBServer", "-device", config.Target.JLinkDevice)
+			if ocdOutput {
+				// Make it clear which output is from the daemon.
+				w := &ColorWriter{
+					Out:    os.Stderr,
+					Prefix: "jlink: ",
+					Color:  TermColorYellow,
+				}
+				daemon.Stdout = w
+				daemon.Stderr = w
+			}
+			// Make sure the daemon doesn't receive Ctrl-C that is intended for
+			// GDB (to break the currently executing program).
+			setCommandAsDaemon(daemon)
+			// Start now, and kill it on exit.
+			daemon.Start()
+			defer func() {
+				daemon.Process.Signal(os.Interrupt)
+				// Maybe we should send a .Kill() after x seconds?
+				daemon.Wait()
+			}()
 		case "qemu":
 			gdbCommands = append(gdbCommands, "target remote :1234")