To judge send ZLP or not in USBCDC::AsyncWrite

Author: cyliangtw

drivers/usb/source/USBCDC.cpp

@@ -39,7 +39,7 @@ class USBCDC::AsyncWrite: public AsyncOp {
     AsyncWrite(USBCDC *serial, uint8_t *buf, uint32_t size):
         serial(serial), tx_buf(buf), tx_size(size), result(false)
     {
-
+        need_zlp = (size % CDC_MAX_PACKET_SIZE == 0) ? true : false;
     }
 
     virtual ~AsyncWrite()
@@ -59,6 +59,12 @@ class USBCDC::AsyncWrite: public AsyncOp {
         tx_size -= actual_size;
         tx_buf += actual_size;
         if (tx_size == 0) {
+            // For ZLP case, not ending yet and need one more time to invoke process to send zero packet.
+            if (need_zlp) {                
+                need_zlp = false;
+                serial->_send_isr_start();
+                return false;
+            }
             result = true;
             return true;
         }
@@ -72,6 +78,7 @@ class USBCDC::AsyncWrite: public AsyncOp {
     uint8_t *tx_buf;
     uint32_t tx_size;
     bool result;
+    bool need_zlp;
 };
 
 class USBCDC::AsyncRead: public AsyncOp {
@@ -388,7 +395,9 @@ void USBCDC::send_nb(uint8_t *buffer, uint32_t size, uint32_t *actual, bool now)
         }
         _tx_size += write_size;
         *actual = write_size;
-        if ((CDC_MAX_PACKET_SIZE == size) && (CDC_MAX_PACKET_SIZE == write_size)) {
+        
+        /* Enable ZLP flag as while send_nb() zero size */
+        if (size == 0) {
             _trans_zlp = true;
         }
 
@@ -409,6 +418,14 @@ void USBCDC::_send_isr_start()
             _tx_in_progress = true;
         }
     }
+
+    /* Send ZLP write start */
+    if (!_tx_in_progress && _trans_zlp) {
+        if (USBDevice::write_start(_bulk_in, _tx_buffer, 0)) {
+            _tx_in_progress = true;
+            _trans_zlp = false;
+        }
+    }
 }
 
 /*
@@ -419,11 +436,6 @@ void USBCDC::_send_isr()
 {
     assert_locked();
 
-    /* Send ZLP write start after last alignment packet sent */
-    if (_trans_zlp && USBDevice::write_start(_bulk_in, _tx_buffer, 0)) {
-        _trans_zlp = false;
-    }
-
     write_finish(_bulk_in);
     _tx_buf = _tx_buffer;
     _tx_size = 0;