Index: head/sys/dev/usb/serial/uftdi.c
===================================================================
--- head/sys/dev/usb/serial/uftdi.c
+++ head/sys/dev/usb/serial/uftdi.c
@@ -1123,6 +1123,9 @@
 	    FTDI_SIO_SET_DATA_PARITY_NONE |
 	    FTDI_SIO_SET_DATA_BITS(8));
 
+	/* Indicate tx bits in sc_lsr can be used to determine busy vs idle. */
+	ucom_use_lsr_txbits(&sc->sc_ucom);
+
 	error = ucom_attach(&sc->sc_super_ucom, &sc->sc_ucom, 1, sc,
 	    &uftdi_callback, &sc->sc_mtx);
 	if (error) {
@@ -1279,16 +1282,20 @@
 		offset = 0;
 		/*
 		 * Extract packet headers and payload bytes from the buffer.
-		 * Feed payload bytes to ucom/tty layer; OR-accumulate header
-		 * status bits which are transient and could toggle with each
-		 * packet. After processing all packets in the buffer, process
-		 * the accumulated transient MSR and LSR values along with the
+		 * Feed payload bytes to ucom/tty layer; OR-accumulate the
+		 * receiver-related header status bits which are transient and
+		 * could toggle with each packet, but for transmitter-related
+		 * bits keep only the ones from the last packet.
+		 *
+		 * After processing all packets in the buffer, process the
+		 * accumulated transient MSR and LSR values along with the
 		 * non-transient bits from the last packet header.
 		 */
 		while (buflen >= UFTDI_IHDRSIZE) {
 			usbd_copy_out(pc, offset, buf, UFTDI_IHDRSIZE);
 			offset += UFTDI_IHDRSIZE;
 			buflen -= UFTDI_IHDRSIZE;
+			lsr &= ~(ULSR_TXRDY | ULSR_TSRE);
 			lsr |= FTDI_GET_LSR(buf);
 			if (FTDI_GET_MSR(buf) & FTDI_SIO_RI_MASK)
 				msr |= SER_RI;
@@ -1311,8 +1318,7 @@
 		if (ftdi_msr & FTDI_SIO_RLSD_MASK)
 			msr |= SER_DCD;
 
-		if ((sc->sc_msr != msr) ||
-		    ((sc->sc_lsr & FTDI_LSR_MASK) != (lsr & FTDI_LSR_MASK))) {
+		if (sc->sc_msr != msr || sc->sc_lsr != lsr) {
 			DPRINTF("status change msr=0x%02x (0x%02x) "
 			    "lsr=0x%02x (0x%02x)\n", msr, sc->sc_msr,
 			    lsr, sc->sc_lsr);
Index: head/sys/dev/usb/serial/usb_serial.h
===================================================================
--- head/sys/dev/usb/serial/usb_serial.h
+++ head/sys/dev/usb/serial/usb_serial.h
@@ -180,6 +180,7 @@
 #define	UCOM_FLAG_WAIT_REFS   0x0100	/* set if we must wait for refs */
 #define	UCOM_FLAG_FREE_UNIT   0x0200	/* set if we must free the unit */
 #define	UCOM_FLAG_INWAKEUP    0x0400	/* set if we are in the tsw_inwakeup callback */
+#define	UCOM_FLAG_LSRTXIDLE   0x0800	/* set if sc_lsr bits ULSR_TSRE+TXRDY work */
 	uint8_t	sc_lsr;
 	uint8_t	sc_msr;
 	uint8_t	sc_mcr;
@@ -218,4 +219,12 @@
 void	ucom_drain_all(void *);
 void	ucom_ref(struct ucom_super_softc *);
 int	ucom_unref(struct ucom_super_softc *);
+
+static inline void
+ucom_use_lsr_txbits(struct ucom_softc *sc)
+{
+
+	sc->sc_flag |= UCOM_FLAG_LSRTXIDLE;
+}
+
 #endif					/* _USB_SERIAL_H_ */
Index: head/sys/dev/usb/serial/usb_serial.c
===================================================================
--- head/sys/dev/usb/serial/usb_serial.c
+++ head/sys/dev/usb/serial/usb_serial.c
@@ -161,6 +161,7 @@
 static tsw_outwakeup_t ucom_outwakeup;
 static tsw_inwakeup_t ucom_inwakeup;
 static tsw_free_t ucom_free;
+static tsw_busy_t ucom_busy;
 
 static struct ttydevsw ucom_class = {
 	.tsw_flags = TF_INITLOCK | TF_CALLOUT,
@@ -172,6 +173,7 @@
 	.tsw_param = ucom_param,
 	.tsw_modem = ucom_modem,
 	.tsw_free = ucom_free,
+	.tsw_busy = ucom_busy,
 };
 
 MODULE_DEPEND(ucom, usb, 1, 1, 1);
@@ -1294,6 +1296,27 @@
 	ucom_start_transfers(sc);
 }
 
+static bool
+ucom_busy(struct tty *tp)
+{
+	struct ucom_softc *sc = tty_softc(tp);
+	const uint8_t txidle = ULSR_TXRDY | ULSR_TSRE;
+
+	UCOM_MTX_ASSERT(sc, MA_OWNED);
+
+	DPRINTFN(3, "sc = %p lsr 0x%02x\n", sc, sc->sc_lsr);
+
+	/*
+	 * If the driver maintains the txidle bits in LSR, we can use them to
+	 * determine whether the transmitter is busy or idle.  Otherwise we have
+	 * to assume it is idle to avoid hanging forever on tcdrain(3).
+	 */
+	if (sc->sc_flag & UCOM_FLAG_LSRTXIDLE)
+		return ((sc->sc_lsr & txidle) != txidle);
+	else
+		return (false);
+}
+
 /*------------------------------------------------------------------------*
  *	ucom_get_data
  *