linux-kernel - [PATCH 3/4] USB: serial: ti_usb_3410_5052: reduce drain delay to one char

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Date:   Mon, 12 Apr 2021 11:38:14 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 3/4] USB: serial: ti_usb_3410_5052: reduce drain delay to one char

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.

Signed-off-by: Johan Hovold <johan@...nel.org>
---
 drivers/usb/serial/ti_usb_3410_5052.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/serial/ti_usb_3410_5052.c b/drivers/usb/serial/ti_usb_3410_5052.c
index 03839289d6c0..8ed64115987f 100644
--- a/drivers/usb/serial/ti_usb_3410_5052.c
+++ b/drivers/usb/serial/ti_usb_3410_5052.c
@@ -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