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esp32: Fix USB Serial/JTAG TX stall due to full EP buffer #19656

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esp32: Fix USB Serial/JTAG TX stall due to full EP buffer #19656

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ae56a38
esp32: Fix USB Serial/jTAG 64-byte tx race triggering stalled input.
projectgus Aug 27, 2026
4815641
tests/serial_test: Allow more time for soft reset of board.
projectgus Aug 27, 2026
6e22f5a
esp32/usb_serial_jtag: Fix tick overflow issue in timeout.
projectgus Aug 27, 2026
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27 changes: 21 additions & 6 deletions ports/esp32/usb_serial_jtag.c
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Original file line number Diff line number Diff line change
Expand Up @@ -77,12 +77,14 @@ static void usb_serial_jtag_isr_handler(void *arg) {
}

if (flags & USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY) {
// As per the ESP-IDF driver, allow for the possibility the USJ just sent a full
// 64-bit endpoint to the host and now it's waiting for another ZLP to flush the result
// to the OS
// This interrupt is only enabled after we've sent a full IN EP buffer to the host.

// As per the ESP-IDF driver, this is necessary if the USJ has just
// sent a full 64-byte EP to the host and now the host is waiting
// for another ZLP to signal that the transfer is actually done.
usb_serial_jtag_ll_txfifo_flush();

// Disable this interrupt until next time we write into the FIFO
// Disable this interrupt until the next time we write a full EP buffer
usb_serial_jtag_ll_disable_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
}
}
Expand Down Expand Up @@ -113,7 +115,7 @@ void usb_serial_jtag_tx_strn(const char *str, size_t len) {
TickType_t start_tick = xTaskGetTickCount();
while (!usb_serial_jtag_ll_txfifo_writable()) {
TickType_t now_tick = xTaskGetTickCount();
if (!terminal_connected || now_tick > (start_tick + pdMS_TO_TICKS(200))) {
if (!terminal_connected || (now_tick - start_tick) > pdMS_TO_TICKS(200)) {
terminal_connected = false;
return;
}
Expand All @@ -122,9 +124,22 @@ void usb_serial_jtag_tx_strn(const char *str, size_t len) {
l = usb_serial_jtag_ll_write_txfifo((const uint8_t *)str, l);
str += l;
len -= l;
}

// Clear any old "serial in empty" interrupt, as we're about to either
// manually flush or enable the interrupt if the TXFIFO is full (no risk of
// a lost flush as we check if the TXFIFO is full after we do this.)
usb_serial_jtag_ll_clr_intsts_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);

if (usb_serial_jtag_ll_txfifo_writable()) {
// If we haven't written a full EP buffer, manually flush it to send to the host
usb_serial_jtag_ll_txfifo_flush();
} else {
// If we have written a full EP buffer to send to the host then the hardware will flush
// automatically, but enable the "serial in empty" interrupt so we will send a ZLP once
// the full buffer has been sent.
usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
}
usb_serial_jtag_ll_txfifo_flush();
}

#endif // MICROPY_HW_ESP_USB_SERIAL_JTAG
33 changes: 28 additions & 5 deletions tests/serial_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,13 @@

from test_utils import test_instance_epilog, convert_device_shortcut_to_real_device

TEST_READ_TIMEOUT = 1

# For ESP32 boards with both native/USJ USB and a UART serial port, soft
# reset may flush the UART buffer before it completes - this can be quite large
# if it's backed up a lot of data during the previous test.
SOFT_RESET_TIMEOUT = 5

echo_test_script = """
import sys
bytes_min=%u
Expand Down Expand Up @@ -98,9 +105,25 @@ def drain_input(ser):
time.sleep(0.1)


def reset_into_raw_repl(ser):
try:
ser.timeout = SOFT_RESET_TIMEOUT
ser.flushInput()
ser.write(b"\x03\x01\x04") # break, raw-repl, soft-reboot
EXPECTED1 = b"MPY: soft reboot\r\n"
EXPECTED2 = b"raw REPL; CTRL-B to exit\r\n>"
r1 = ser.read_until(EXPECTED1)
r2 = b""
if r1.endswith(EXPECTED1):
r2 = ser.read_until(EXPECTED2)
if not r2.endswith(EXPECTED2):
raise TestError("could not reset to raw REPL", r1 + r2)
finally:
ser.timeout = TEST_READ_TIMEOUT


def send_script(ser, script):
ser.write(b"\x03\x01\x04") # break, raw-repl, soft-reboot
drain_input(ser)
reset_into_raw_repl(ser)
chunk_size = 32
for i in range(0, len(script), chunk_size):
ser.write(script[i : i + chunk_size])
Expand Down Expand Up @@ -270,13 +293,13 @@ def write_test(ser_repl, ser_data, bufsize, nbuf, verified):
def do_test(dev_repl, dev_data=None, time_per_subtest=1):
if dev_data is None:
print("REPL and data on", dev_repl)
ser_repl = serial.Serial(dev_repl, baudrate=115200, timeout=1)
ser_repl = serial.Serial(dev_repl, baudrate=115200, timeout=TEST_READ_TIMEOUT)
ser_data = ser_repl
else:
print("REPL on", dev_repl)
print("data on", dev_data)
ser_repl = serial.Serial(dev_repl, baudrate=115200, timeout=1)
ser_data = serial.Serial(dev_data, baudrate=115200, timeout=1)
ser_repl = serial.Serial(dev_repl, baudrate=115200, timeout=TEST_READ_TIMEOUT)
ser_data = serial.Serial(dev_data, baudrate=115200, timeout=TEST_READ_TIMEOUT)

# Do echo test first, and abort if it doesn't pass.
echo_test(ser_repl, ser_data)
Expand Down
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