USB: serial: closing-wait fixes and cleanups

The port drain_delay parameter is used to add a time-based delay when closing the port in order to allow the transmit FIFO to drain in cases where we don't know how to tell if the FIFO is empty.

This series removes a redundant time-based delay which is no longer needed, and documents the reason for two other uses where such a delay is needed to let the transmitter shift register clear. As it turns out, this is really only needed for one of the two device types handled by the ti_usb_3410_5052 driver.

Johan

Johan Hovold (4):
USB: serial: f81232: drop time-based drain delay
USB: serial: io_ti: document reason for drain delay
USB: serial: ti_usb_3410_5052: reduce drain delay to one char
USB: serial: ti_usb_3410_5052: drop drain delay for 3410

 drivers/usb/serial/f81232.c           |  1 -
 drivers/usb/serial/io_ti.c            |  4 ++++
 drivers/usb/serial/ti_usb_3410_5052.c | 21 ++++++++++++++++++---
 3 files changed, 22 insertions(+), 4 deletions(-)

2.26.3

The f81232 driver now waits for the transmit FIFO to drain during close so there is no need to keep the time-based drain delay, which would add up to two seconds on every close for low line speeds.

---
 drivers/usb/serial/f81232.c | 1 -
 1 file changed, 1 deletion(-)

index b4b847dce4bc..a7a7af8d05bf 100644
@@ -948,7 +948,6 @@ static int f81232_port_probe(struct usb_serial_port *port)
 
 	usb_set_serial_port_data(port, priv);
 
-	port->port.drain_delay = 256;
 	priv->port = port;
 
 	return 0;

2.26.3

Document that the device line-status register doesn't tell when the transmitter shift register has emptied and that this is why the one-character drain delay is needed.

---
 drivers/usb/serial/io_ti.c | 4 ++++
 1 file changed, 4 insertions(+)

index 75325c2b295e..17720670e06c 100644
@@ -2590,6 +2590,10 @@ static int edge_port_probe(struct usb_serial_port *port)
 	if (ret)
 		goto err;
 
+	/*
+	 * The LSR does not tell when the transmitter shift register has
+	 * emptied so add a one-character drain delay.
+	 */
 	port->port.drain_delay = 1;
 
 	return 0;

2.26.3

The three-character drain delay was added by commit f1175daa5312 ("USB:
ti_usb_3410_5052: kill custom closing_wait") when removing the custom closing-wait implementation, which used a fixed 20 ms poll period and drain delay.

This was likely a bit too conservative as a one-character timeout (e.g. 33 ms at 300 bps) should be enough to compensate for the lack of a transmitter empty bit in the TUSB5052 line-status register.

---
 drivers/usb/serial/ti_usb_3410_5052.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

index 03839289d6c0..8ed64115987f 100644
@@ -610,7 +610,11 @@ static int ti_port_probe(struct usb_serial_port *port)
 
 	usb_set_serial_port_data(port, tport);
 
-	port->port.drain_delay = 3;
+	/*
+	 * The TUSB5052 LSR does not tell when the transmitter shift register
+	 * has emptied so add a one-character drain delay.
+	 */
+	port->port.drain_delay = 1;
 
 	return 0;
 }

2.26.3

Unlike the TUSB5052, the TUSB3410 has an LSR TEMT bit to tell if both the transmitter data and shift registers are empty.

Make sure to check also the shift register on TUSB3410 when waiting for the transmit buffer to drain during close and drop the time-based one-char delay which is otherwise needed (e.g. 90 ms at 110 bps).

---
 drivers/usb/serial/ti_usb_3410_5052.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

index 8ed64115987f..d9bffb2de8bf 100644
@@ -121,6 +121,7 @@
 #define TI_LSR_ERROR			0x0F
 #define TI_LSR_RX_FULL			0x10
 #define TI_LSR_TX_EMPTY			0x20
+#define TI_LSR_TX_EMPTY_BOTH		0x40
 
 /* Line control */
 #define TI_LCR_BREAK			0x40

@@ -614,7 +615,8 @@ static int ti_port_probe(struct usb_serial_port *port)
 	 * The TUSB5052 LSR does not tell when the transmitter shift register
 	 * has emptied so add a one-character drain delay.
 	 */
-	port->port.drain_delay = 1;
+	if (!tport->tp_tdev->td_is_3410)
+		port->port.drain_delay = 1;
 
 	return 0;
 }

@@ -851,11 +853,20 @@ static int ti_chars_in_buffer(struct tty_struct *tty)
 static bool ti_tx_empty(struct usb_serial_port *port)
 {
 	struct ti_port *tport = usb_get_serial_port_data(port);
+	u8 lsr, mask;
 	int ret;
-	u8 lsr;
+
+	/*
+	 * TUSB5052 does not have the TEMT bit to tell if the shift register
+	 * is empty.
+	 */
+	if (tport->tp_tdev->td_is_3410)
+		mask = TI_LSR_TX_EMPTY_BOTH;
+	else
+		mask = TI_LSR_TX_EMPTY;
 
 	ret = ti_get_lsr(tport, &lsr);
-	if (!ret && !(lsr & TI_LSR_TX_EMPTY))
+	if (!ret && !(lsr & mask))
 		return false;
 
 	return true;

2.26.3