shared/tinyusb: Fix hang from new tx_overwritabe_if_not_connected flag.

projectgus · projectgus · commit e29b383fc95f · 2025-08-28T14:32:52.000+10:00
This flag is in the main branch of TinyUSB, included in Espressif since
their v0.18.0~3 component release (but it's not actually in TinyUSB v0.18.0
release).

Setting the flag is needed for the USB device not to block waiting for
space in the FIFO if the host is disconnected.

Running mp_usbd_task() unconditionally in the loop may not be necessary,
but avoids the risk of a race condition where tud_cdc_connected() returns
true, the FIFO is in blocking mode, but the host has actually
disconnected (it just hasn't been processed by TinyUSB task yet).

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton &lt;angus@redyak.com.au&gt;
diff --git a/shared/tinyusb/mp_usbd.h b/shared/tinyusb/mp_usbd.h
@@ -43,7 +43,18 @@
 // Initialise TinyUSB device.
 static inline void mp_usbd_init_tud(void) {
     tusb_init();
-    tud_cdc_configure_fifo_t cfg = { .rx_persistent = 0, .tx_persistent = 1 };
+    tud_cdc_configure_fifo_t cfg = { .rx_persistent = 0,
+                                     .tx_persistent = 1,
+
+                                     // This config flag is unreleased in TinyUSB >v0.18.0
+                                     // but included in Espressif's TinyUSB component since v0.18.0~3
+                                     //
+                                     // Versioning issue reported as
+                                     // https://github.com/espressif/esp-usb/issues/236
+                                     #if TUSB_VERSION_NUMBER > 1800 || defined(ESP_PLATFORM)
+                                     .tx_overwritabe_if_not_connected = 1,
+                                     #endif
+    };
     tud_cdc_configure_fifo(&cfg);
 }
 
diff --git a/shared/tinyusb/mp_usbd_cdc.c b/shared/tinyusb/mp_usbd_cdc.c
@@ -98,32 +98,39 @@ mp_uint_t mp_usbd_cdc_tx_strn(const char *str, mp_uint_t len) {
     if (!tusb_inited()) {
         return 0;
     }
+    mp_uint_t last_write = mp_hal_ticks_ms();
     size_t i = 0;
-    while (i < len) {
+    while (i < len && (mp_uint_t)(mp_hal_ticks_ms() - last_write) < MICROPY_HW_USB_CDC_TX_TIMEOUT) {
         uint32_t n = len - i;
-        if (n > CFG_TUD_CDC_EP_BUFSIZE) {
-            n = CFG_TUD_CDC_EP_BUFSIZE;
-        }
-        if (tud_cdc_connected()) {
-            // If CDC port is connected but the buffer is full, wait for up to USC_CDC_TIMEOUT ms.
-            mp_uint_t t0 = mp_hal_ticks_ms();
-            while (n > tud_cdc_write_available() && (mp_uint_t)(mp_hal_ticks_ms() - t0) < MICROPY_HW_USB_CDC_TX_TIMEOUT) {
-                mp_event_wait_ms(1);
 
-                // Explicitly run the USB stack as the scheduler may be locked (eg we
-                // are in an interrupt handler), while there is data pending.
-                mp_usbd_task();
-            }
+        if (tud_cdc_connected()) {
             // Limit write to available space in tx buffer when connected.
+            // (Otherwise we always write to usb fifo, expecting it to overwrite
+            // old data so it will have latest data buffered when host connects.)
             n = MIN(n, tud_cdc_write_available());
-            if (n == 0) {
-                break;
-            }
         }
-        // When not connected we always write to usb fifo, ensuring it has latest data.
+
         uint32_t n2 = tud_cdc_write(str + i, n);
         tud_cdc_write_flush();
         i += n2;
+
+        if (i < len) {
+            if (n2 > 0) {
+                // reset the timeout each time we successfully write to the FIFO
+                last_write = mp_hal_ticks_ms();
+            }
+
+            if (tud_cdc_connected()) {
+                // If we know we're connected then we can wait for host to make
+                // more space
+                mp_event_wait_ms(1);
+            }
+
+            // Always explicitly run the USB stack as the scheduler may be
+            // locked (eg we are in an interrupt handler), while there is data
+            // or a state change pending.
+            mp_usbd_task();
+        }
     }
     return i;
 }
