stm32/usbd_cdc_interface: Don't retransmit chars if USB is reconnected.

Before this change, if the USB was reconnected it was possible that some
characters in the TX buffer were retransmitted because tx_buf_ptr_out and
tx_buf_ptr_out_shadow were reset while tx_buf_ptr_in wasn't.  That
behaviour is fixed here by retaining the TX buffer state across reconnects.

Fixes issue #4761.

2 files changed, 6 insertions, 6 deletions

diff --git a/ports/stm32/usb.c b/ports/stm32/usb.c
index 18841fdee..8b7608a15 100644
--- a/ports/stm32/usb.c
+++ b/ports/stm32/usb.c
@@ -69,7 +69,7 @@ typedef struct _usb_device_t {
     usbd_hid_itf_t usbd_hid_itf;
 } usb_device_t;
 
-usb_device_t usb_device;
+usb_device_t usb_device = {0};
 pyb_usb_storage_medium_t pyb_usb_storage_medium = PYB_USB_STORAGE_MEDIUM_NONE;
 
 // predefined hid mouse data
diff --git a/ports/stm32/usbd_cdc_interface.c b/ports/stm32/usbd_cdc_interface.c
index 586f2d525..4a4a8beb8 100644
--- a/ports/stm32/usbd_cdc_interface.c
+++ b/ports/stm32/usbd_cdc_interface.c
@@ -64,14 +64,14 @@ static uint8_t usbd_cdc_connect_tx_timer;
 uint8_t *usbd_cdc_init(usbd_cdc_state_t *cdc_in) {
     usbd_cdc_itf_t *cdc = (usbd_cdc_itf_t*)cdc_in;
 
-    // Reset all the CDC state
-    // Note: we don't reset tx_buf_ptr_in in order to allow the output buffer to
-    // be filled (by usbd_cdc_tx_always) before the USB device is connected.
+    // Reset the CDC state due to a new USB host connection
+    // Note: we don't reset tx_buf_ptr_* in order to allow the output buffer to
+    // be filled (by usbd_cdc_tx_always) before the USB device is connected, and
+    // to retain transmit buffer state across multiple USB connections (they will
+    // be 0 at MCU reset since the variables live in the BSS).
     cdc->rx_buf_put = 0;
     cdc->rx_buf_get = 0;
     cdc->rx_buf_full = false;
-    cdc->tx_buf_ptr_out = 0;
-    cdc->tx_buf_ptr_out_shadow = 0;
     cdc->tx_need_empty_packet = 0;
     cdc->connect_state = USBD_CDC_CONNECT_STATE_DISCONNECTED;