feat: add ResetAuto mode for automatic reset strategy detection

- rotate through classic DTR/RTS, USB-JTAG, and no-reset strategies
- add ResetMode.String() method
- extend hardReset with USB timing support

Author: jgangemi

pkg/espflasher/flasher.go

@@ -211,6 +211,15 @@ func (f *Flasher) connect() error {
 			}
 		case ResetUSBJTAG:
 			usbJTAGSerialReset(f.port)
+		case ResetAuto:
+			switch attempt % 3 {
+			case 0:
+				classicReset(f.port, defaultResetDelay)
+			case 1:
+				usbJTAGSerialReset(f.port)
+			case 2:
+				// No reset — device may already be in bootloader
+			}
 		}
 		// ResetNoReset: skip reset entirely
 

pkg/espflasher/flasher_test.go

@@ -8,6 +8,28 @@ import (
 	"testing"
 )
 
+func TestResetModeString(t *testing.T) {
+	tests := []struct {
+		mode ResetMode
+		want string
+	}{
+		{ResetDefault, "default"},
+		{ResetNoReset, "no-reset"},
+		{ResetUSBJTAG, "usb-jtag"},
+		{ResetAuto, "auto"},
+		{ResetMode(99), "unknown(99)"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.want, func(t *testing.T) {
+			got := tt.mode.String()
+			if got != tt.want {
+				t.Errorf("String() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
 func TestDefaultOptions(t *testing.T) {
 	opts := DefaultOptions()
 

pkg/espflasher/reset.go

@@ -1,6 +1,7 @@
 package espflasher
 
 import (
+	"fmt"
 	"time"
 
 	"go.bug.st/serial"
@@ -19,6 +20,11 @@ const (
 
 	// ResetUSBJTAG uses the USB-JTAG/Serial reset sequence (ESP32-S3, ESP32-C3, etc.).
 	ResetUSBJTAG
+
+	// ResetAuto tries multiple reset strategies in sequence.
+	// First attempts DTR/RTS classic reset, then USB-JTAG, then no-signal.
+	// Useful when the interface type is unknown.
+	ResetAuto
 )
 
 const (
@@ -112,3 +118,19 @@ func hardReset(port serial.Port, usesUSB bool) {
 		port.SetRTS(false) //nolint:errcheck
 	}
 }
+
+// String returns the string representation of the ResetMode.
+func (r ResetMode) String() string {
+	switch r {
+	case ResetDefault:
+		return "default"
+	case ResetNoReset:
+		return "no-reset"
+	case ResetUSBJTAG:
+		return "usb-jtag"
+	case ResetAuto:
+		return "auto"
+	default:
+		return fmt.Sprintf("unknown(%d)", int(r))
+	}
+}