linux-kernel - [PATCH AUTOSEL 4.9 04/13] usb: gadget: serial: fix Tx stall after buffer overflow

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Date:   Mon,  2 Mar 2020 21:49:53 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Sergey Organov <sorganov@...il.com>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Felipe Balbi <balbi@...nel.org>,
        Sasha Levin <sashal@...nel.org>, linux-usb@...r.kernel.org
Subject: [PATCH AUTOSEL 4.9 04/13] usb: gadget: serial: fix Tx stall after buffer overflow

From: Sergey Organov <sorganov@...il.com>

[ Upstream commit e4bfded56cf39b8d02733c1e6ef546b97961e18a ]

Symptom: application opens /dev/ttyGS0 and starts sending (writing) to
it while either USB cable is not connected, or nobody listens on the
other side of the cable. If driver circular buffer overflows before
connection is established, no data will be written to the USB layer
until/unless /dev/ttyGS0 is closed and re-opened again by the
application (the latter besides having no means of being notified about
the event of establishing of the connection.)

Fix: on open and/or connect, kick Tx to flush circular buffer data to
USB layer.

Signed-off-by: Sergey Organov <sorganov@...il.com>
Reviewed-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
Signed-off-by: Felipe Balbi <balbi@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/usb/gadget/function/u_serial.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index 510a54f889635..5d7d0f2e80a5e 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -715,8 +715,10 @@ static int gs_start_io(struct gs_port *port)
 	port->n_read = 0;
 	started = gs_start_rx(port);
 
-	/* unblock any pending writes into our circular buffer */
 	if (started) {
+		gs_start_tx(port);
+		/* Unblock any pending writes into our circular buffer, in case
+		 * we didn't in gs_start_tx() */
 		tty_wakeup(port->port.tty);
 	} else {
 		gs_free_requests(ep, head, &port->read_allocated);
-- 
2.20.1