USB: gadget: g_serial: append zlp when tx buffer becomes empty

Some usb serial host drivers expect a short packet before they forward
the data to the application. This is caused by them trying to read more
than one packet at a time. So when the gadget sends an exact multiple
of the maximum packet size, it should append a zero-length packet.

Signed-off-by: Daniel Glöckner <dg@emlix.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/usb/gadget/u_serial.c b/drivers/usb/gadget/u_serial.c
index 0a4d99ab40d8..fc6e709f45b1 100644
--- a/drivers/usb/gadget/u_serial.c
+++ b/drivers/usb/gadget/u_serial.c
@@ -371,6 +371,7 @@ __acquires(&port->port_lock)
 
                 req->length = len;
                 list_del(&req->list);
+                req->zero = (gs_buf_data_avail(&port->port_write_buf) == 0);
 
                 pr_vdebug(PREFIX "%d: tx len=%d, 0x%02x 0x%02x 0x%02x ...\n",
                                 port->port_num, len, *((u8 *)req->buf),