[PATCH 4.4 42/72] usb: gadget: serial: fix Tx stall after buffer overflow

[Greg Kroah-Hartman]

From: Sergey Organov <sorganov@xxxxxxxxx>

[ 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@xxxxxxxxx>
Reviewed-by: MichaÅ MirosÅaw <mirq-linux@xxxxxxxxxxxx>
Signed-off-by: Felipe Balbi <balbi@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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 31e08bb3cb41e..58a699cfa4582 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -701,8 +701,10 @@ 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