6 files changed, 35 insertions, 0 deletions
diff --git a/examples/device/webusb_serial/src/main.c b/examples/device/webusb_serial/src/main.c
index 50794bdba..155768b76 100644
--- a/examples/device/webusb_serial/src/main.c
+++ b/examples/device/webusb_serial/src/main.c
@@ -196,6 +196,7 @@ bool tud_vendor_control_xfer_cb(uint8_t rhport, uint8_t stage, tusb_control_requ
           tud_vendor_write_str("\r\nWebUSB interface connected\r\n");
           tud_vendor_write_flush();
         } else {
+          tud_vendor_write_clear(); // anything left in the buffer is now thrown out
           blink_interval_ms = BLINK_MOUNTED;
         }
 
diff --git a/src/class/vendor/vendor_device.c b/src/class/vendor/vendor_device.c
index 9972911e0..ced3c0d71 100644
--- a/src/class/vendor/vendor_device.c
+++ b/src/class/vendor/vendor_device.c
@@ -160,6 +160,12 @@ uint32_t tud_vendor_n_write_available(uint8_t idx) {
   vendord_interface_t *p_itf = &_vendord_itf[idx];
   return tu_edpt_stream_write_available(&p_itf->stream.tx);
 }
+
+uint32_t tud_vendor_n_write_clear(uint8_t idx) {
+  TU_VERIFY(idx < CFG_TUD_VENDOR, 0);
+  vendord_interface_t *p_itf = &_vendord_itf[idx];
+  return tu_edpt_stream_count_and_clear(&p_itf->stream.tx);
+}
 #endif
 
 //--------------------------------------------------------------------+
diff --git a/src/class/vendor/vendor_device.h b/src/class/vendor/vendor_device.h
index d59c885d2..878d7aee0 100644
--- a/src/class/vendor/vendor_device.h
+++ b/src/class/vendor/vendor_device.h
@@ -91,6 +91,9 @@ uint32_t tud_vendor_n_write_flush(uint8_t idx);
 
 // Return number of bytes available for writing in TX FIFO
 uint32_t tud_vendor_n_write_available(uint8_t idx);
+
+// Clear the write buffer and return the number of elements cleared
+uint32_t tud_vendor_n_write_clear(uint8_t idx);
 #endif
 
 // Write a null-terminated string to TX FIFO
@@ -148,6 +151,10 @@ TU_ATTR_ALWAYS_INLINE static inline uint32_t tud_vendor_write_flush(void) {
 TU_ATTR_ALWAYS_INLINE static inline uint32_t tud_vendor_write_available(void) {
   return tud_vendor_n_write_available(0);
 }
+
+TU_ATTR_ALWAYS_INLINE static inline uint32_t tud_vendor_write_clear(void) {
+  return tud_vendor_n_write_clear(0);
+}
 #endif
 
 // backward compatible
diff --git a/src/common/tusb_fifo.c b/src/common/tusb_fifo.c
index e97b6ccce..28086be0b 100644
--- a/src/common/tusb_fifo.c
+++ b/src/common/tusb_fifo.c
@@ -95,6 +95,22 @@ void tu_fifo_clear(tu_fifo_t *f) {
   ff_unlock(f->mutex_rd);
 }
 
+// synchronously count then clear fifo, returning the count
+uint16_t tu_fifo_count_and_clear(tu_fifo_t *f) {
+  ff_lock(f->mutex_wr);
+  ff_lock(f->mutex_rd);
+
+  uint16_t cnt = tu_fifo_count(f);
+
+  f->rd_idx = 0;
+  f->wr_idx = 0;
+
+  ff_unlock(f->mutex_wr);
+  ff_unlock(f->mutex_rd);
+
+  return cnt;
+}
+
 // Change the fifo overwritable mode
 void tu_fifo_set_overwritable(tu_fifo_t *f, bool overwritable) {
   if (f->overwritable == overwritable) {
diff --git a/src/common/tusb_fifo.h b/src/common/tusb_fifo.h
index 2e2a0db6f..75eeded01 100644
--- a/src/common/tusb_fifo.h
+++ b/src/common/tusb_fifo.h
@@ -159,6 +159,7 @@ typedef enum {
 bool tu_fifo_config(tu_fifo_t *f, void *buffer, uint16_t depth, uint16_t item_size, bool overwritable);
 void tu_fifo_set_overwritable(tu_fifo_t *f, bool overwritable);
 void tu_fifo_clear(tu_fifo_t *f);
+uint16_t tu_fifo_count_and_clear(tu_fifo_t *f);
 
 #if OSAL_MUTEX_REQUIRED
 TU_ATTR_ALWAYS_INLINE static inline
diff --git a/src/common/tusb_private.h b/src/common/tusb_private.h
index 10e12c2af..0c1709d49 100644
--- a/src/common/tusb_private.h
+++ b/src/common/tusb_private.h
@@ -144,6 +144,10 @@ TU_ATTR_ALWAYS_INLINE static inline void tu_edpt_stream_clear(tu_edpt_stream_t *
   tu_fifo_clear(&s->ff);
 }
 
+TU_ATTR_ALWAYS_INLINE static inline uint16_t tu_edpt_stream_count_and_clear(tu_edpt_stream_t *s) {
+  return tu_fifo_count_and_clear(&s->ff);
+}
+
 TU_ATTR_ALWAYS_INLINE static inline bool tu_edpt_stream_empty(tu_edpt_stream_t *s) {
   return tu_fifo_empty(&s->ff);
 }