feat: add USB CDC support, ReadFlash protocol, and ESP32-S3 post-connect

- chunk SLIP writes to 64 bytes for USB CDC endpoint compatibility
- use 1KB RAM upload blocks for USB connections
- detect USB-OTG and USB-JTAG/Serial interfaces on ESP32-S3
- disable RTC WDT and enable SWD auto-feed for USB-JTAG/Serial
- add ReadFlash protocol with SLIP-framed blocks and cumulative ACKs
- add SLIP leftover buffer for multi-frame USB transfers
- add port re-open recovery for TinyUSB CDC re-enumeration

Author: jgangemi

pkg/espflasher/chip.go

@@ -117,6 +117,11 @@ type chipDef struct {
 
 	// FlashSizes maps size strings to header byte values.
 	FlashSizes map[string]byte
+
+	// PostConnect is called after chip detection to perform chip-specific
+	// initialization (e.g. USB interface detection, watchdog disable).
+	// May set Flasher fields like usesUSB.
+	PostConnect func(f *Flasher) error
 }
 
 // chipDetectMagicRegAddr is the register address that has a different

pkg/espflasher/flasher.go

@@ -105,20 +105,23 @@ type connection interface {
 	changeBaud(newBaud, oldBaud uint32) error
 	eraseFlash() error
 	eraseRegion(offset, size uint32) error
+	readFlash(offset, size uint32) ([]byte, error)
 	flushInput()
 	isStub() bool
+	setUSB(v bool)
 	setSupportsEncryptedFlash(v bool)
 	loadStub(s *stub) error
 }
 
 // Flasher manages the connection to an ESP device and provides
 // high-level flash operations.
 type Flasher struct {
-	port    serial.Port
-	conn    connection
-	chip    *chipDef
-	opts    *FlasherOptions
-	portStr string
+	port     serial.Port
+	conn     connection
+	chip     *chipDef
+	opts     *FlasherOptions
+	portStr  string
+	usesUSB  bool
 }
 
 // New creates a new Flasher connected to the given serial port.
@@ -162,7 +165,7 @@ func New(portName string, opts *FlasherOptions) (*Flasher, error) {
 
 	// Connect to the bootloader
 	if err := f.connect(); err != nil {
-		port.Close() //nolint:errcheck
+		f.port.Close() //nolint:errcheck
 		return nil, err
 	}
 
@@ -174,6 +177,31 @@ func (f *Flasher) Close() error {
 	return f.port.Close()
 }
 
+// reopenPort closes and reopens the serial port after a USB device
+// re-enumeration. TinyUSB CDC devices may briefly disappear during reset.
+func (f *Flasher) reopenPort() error {
+	f.port.Close() //nolint:errcheck
+
+	var lastErr error
+	deadline := time.Now().Add(3 * time.Second)
+	for time.Now().Before(deadline) {
+		time.Sleep(500 * time.Millisecond)
+		port, err := serial.Open(f.portStr, &serial.Mode{
+			BaudRate: f.opts.BaudRate,
+			Parity:   serial.NoParity,
+			DataBits: 8,
+			StopBits: serial.OneStopBit,
+		})
+		if err == nil {
+			f.port = port
+			f.conn = newConn(port)
+			return nil
+		}
+		lastErr = err
+	}
+	return fmt.Errorf("reopen port %s: %w", f.portStr, lastErr)
+}
+
 // ChipType returns the detected chip type.
 func (f *Flasher) ChipType() ChipType {
 	if f.chip != nil {
@@ -224,6 +252,11 @@ func (f *Flasher) connect() error {
 			}
 			time.Sleep(50 * time.Millisecond)
 		}
+
+		// Sync failed — try reopening port (USB CDC may have re-enumerated)
+		if err := f.reopenPort(); err != nil {
+			continue // port reopen failed, try next attempt
+		}
 	}
 
 	return &SyncError{Attempts: attempts}
@@ -248,9 +281,21 @@ synced:
 
 	f.logf("Detected chip: %s", f.chip.Name)
 
+	// Run chip-specific post-connect initialization.
+	if f.chip.PostConnect != nil {
+		if err := f.chip.PostConnect(f); err != nil {
+			f.logf("Warning: post-connect: %v", err)
+		}
+	}
+
 	// Propagate chip capabilities to the connection layer.
 	f.conn.setSupportsEncryptedFlash(f.chip.SupportsEncryptedFlash)
 
+	// Propagate USB flag to connection layer for block size optimization.
+	if f.usesUSB {
+		f.conn.setUSB(true)
+	}
+
 	// Upload the stub loader to enable advanced features (erase, compression, etc.).
 	if s, ok := stubFor(f.chip.ChipType); ok {
 		f.logf("Loading stub loader...")
@@ -679,6 +724,20 @@ func (f *Flasher) WriteRegister(addr, value uint32) error {
 	return f.conn.writeReg(addr, value, 0xFFFFFFFF, 0)
 }
 
+// ReadFlash reads data from flash memory.
+// Requires the stub loader to be running.
+func (f *Flasher) ReadFlash(offset, size uint32) ([]byte, error) {
+	if !f.conn.isStub() {
+		return nil, &UnsupportedCommandError{Command: "read flash (requires stub)"}
+	}
+
+	if err := f.attachFlash(); err != nil {
+		return nil, err
+	}
+
+	return f.conn.readFlash(offset, size)
+}
+
 // Reset performs a hard reset of the device, causing it to run user code.
 func (f *Flasher) Reset() {
 	if f.conn.isStub() {
@@ -693,7 +752,7 @@ func (f *Flasher) Reset() {
 	// CMD_FLASH_BEGIN after a compressed download may interfere with
 	// the flash controller state at offset 0. esptool also just does
 	// a hard reset without any flash commands for the ROM path.
-	hardReset(f.port, false)
+	hardReset(f.port, f.usesUSB)
 	f.logf("Device reset.")
 }
 

pkg/espflasher/flasher_test.go

@@ -411,3 +411,18 @@ func TestFlashSizeFromJEDECMatchesChipSizes(t *testing.T) {
 		}
 	}
 }
+
+func TestReadFlashRequiresStub(t *testing.T) {
+	mock := &mockConnection{}
+	mock.stubMode = false // ROM mode
+	f := &Flasher{conn: mock, chip: chipDefs[ChipESP32]}
+	_, err := f.ReadFlash(0, 1024)
+	if err == nil {
+		t.Fatal("expected error when stub is not running")
+	}
+	if ue, ok := err.(*UnsupportedCommandError); !ok {
+		t.Errorf("expected UnsupportedCommandError, got %T: %v", err, err)
+	} else if ue.Command != "read flash (requires stub)" {
+		t.Errorf("unexpected error message: %s", ue.Command)
+	}
+}

pkg/espflasher/protocol.go

@@ -59,6 +59,9 @@ const (
 	// flashSectorSize is the minimum flash erase unit.
 	flashSectorSize uint32 = 0x1000 // 4KB
 
+	// readFlashBlockSize is the block size for read flash operations.
+	readFlashBlockSize uint32 = 0x1000 // 4KB
+
 	// espImageMagic is the first byte of a valid ESP firmware image.
 	espImageMagic byte = 0xE9
 
@@ -79,7 +82,8 @@ const (
 type conn struct {
 	port   serial.Port
 	reader *slipReader
-	stub   bool
+	stub    bool
+	usesUSB bool // set for USB-OTG and USB-JTAG/Serial connections
 	// supportsEncryptedFlash indicates the ROM supports the 5th parameter
 	// (encrypted flag) in flash_begin/flash_defl_begin commands.
 	// Set based on chip type after detection.
@@ -91,6 +95,11 @@ func (c *conn) isStub() bool {
 	return c.stub
 }
 
+// setUSB sets whether the connection uses USB-OTG or USB-JTAG endpoints.
+func (c *conn) setUSB(v bool) {
+	c.usesUSB = v
+}
+
 // setSupportsEncryptedFlash sets whether the ROM supports encrypted flash commands.
 func (c *conn) setSupportsEncryptedFlash(v bool) {
 	c.supportsEncryptedFlash = v
@@ -125,8 +134,20 @@ func (c *conn) sendCommand(opcode byte, data []byte, chk uint32) error {
 	copy(pkt[8:], data)
 
 	frame := slipEncode(pkt)
-	_, err := c.port.Write(frame)
-	return err
+	// USB CDC endpoints have limited buffer sizes. Writing large SLIP frames
+	// in one shot can overflow the endpoint buffer and cause data loss.
+	// Chunk writes to 64 bytes (standard USB Full Speed bulk endpoint size).
+	const maxChunk = 64
+	for off := 0; off < len(frame); off += maxChunk {
+		end := off + maxChunk
+		if end > len(frame) {
+			end = len(frame)
+		}
+		if _, err := c.port.Write(frame[off:end]); err != nil {
+			return err
+		}
+	}
+	return nil
 }
 
 // commandResponse represents a parsed response from the ESP device.
@@ -541,6 +562,51 @@ func (c *conn) eraseRegion(offset, size uint32) error {
 	return err
 }
 
+// readFlash reads data from flash memory (stub-only).
+func (c *conn) readFlash(offset, size uint32) ([]byte, error) {
+	data := make([]byte, 16)
+	binary.LittleEndian.PutUint32(data[0:4], offset)
+	binary.LittleEndian.PutUint32(data[4:8], size)
+	binary.LittleEndian.PutUint32(data[8:12], readFlashBlockSize)
+	binary.LittleEndian.PutUint32(data[12:16], 64) // max_inflight (stub clamps to 1)
+
+	if _, err := c.checkCommand("read flash", cmdReadFlash, data, 0, defaultTimeout, 0); err != nil {
+		return nil, err
+	}
+
+	blockTimeout := defaultTimeout + time.Duration(readFlashBlockSize/256)*100*time.Millisecond
+	numBlocks := (size + readFlashBlockSize - 1) / readFlashBlockSize
+	result := make([]byte, 0, size)
+
+	for i := uint32(0); i < numBlocks; i++ {
+		// Read SLIP-framed data block
+		block, err := c.reader.ReadFrame(blockTimeout)
+		if err != nil {
+			return nil, fmt.Errorf("read flash block %d/%d: %w", i+1, numBlocks, err)
+		}
+		result = append(result, block...)
+
+		// Send ACK: cumulative bytes received (SLIP-framed)
+		ack := make([]byte, 4)
+		binary.LittleEndian.PutUint32(ack, uint32(len(result)))
+		ackFrame := slipEncode(ack)
+		if _, err := c.port.Write(ackFrame); err != nil {
+			return nil, fmt.Errorf("read flash ACK %d/%d: %w", i+1, numBlocks, err)
+		}
+	}
+
+	// Read final 16-byte MD5 digest (SLIP-framed)
+	_, err := c.reader.ReadFrame(defaultTimeout)
+	if err != nil {
+		return nil, fmt.Errorf("read flash MD5: %w", err)
+	}
+
+	if uint32(len(result)) > size {
+		result = result[:size]
+	}
+	return result, nil
+}
+
 // flashWriteSize returns the appropriate block size based on loader type.
 func (c *conn) flashWriteSize() uint32 {
 	if c.stub {
@@ -601,17 +667,24 @@ func (c *conn) loadStub(s *stub) error {
 
 // uploadToRAM writes a binary segment to the device's RAM via mem_begin/mem_data.
 func (c *conn) uploadToRAM(data []byte, addr uint32) error {
+	// USB CDC endpoints have limited buffer sizes. Use 1KB blocks for
+	// USB connections instead of the default 6KB to avoid timeout.
+	blockSize := espRAMBlock
+	if c.usesUSB {
+		blockSize = 0x400 // 1KB
+	}
+
 	dataLen := uint32(len(data))
-	numBlocks := (dataLen + espRAMBlock - 1) / espRAMBlock
+	numBlocks := (dataLen + blockSize - 1) / blockSize
 
-	if err := c.memBegin(dataLen, numBlocks, espRAMBlock, addr); err != nil {
+	if err := c.memBegin(dataLen, numBlocks, blockSize, addr); err != nil {
 		return err
 	}
 
 	seq := uint32(0)
 	offset := uint32(0)
 	for offset < dataLen {
-		end := offset + espRAMBlock
+		end := offset + blockSize
 		if end > dataLen {
 			end = dataLen
 		}