[PATCH v2 2/2] usb: gadget: u_serial: implement flush_buffer

[Mehmet Fide <mehmet.fide@gmail.com>]

From: Mehmet Fide <mehmet.fide@screeningeagle.com>

gs_close() waits up to GS_CLOSE_TIMEOUT (15 s) for the write buffer
to drain while the gadget is connected. When the host does not read
the port, for example because no application on the host has it open,
the last close blocks for the full timeout. tcflush(TCOFLUSH) cannot
avoid that: u_serial has no flush_buffer operation, so the request is
a no-op.

Implement flush_buffer: discard the buffered output under the port
lock and wake up the close and write waiters. As on other tty drivers,
a hangup and an interrupt character (ISIG without NOFLSH) now discard
the pending output as well.

Signed-off-by: Mehmet Fide <mehmet.fide@screeningeagle.com>
---

Notes:
    Changes in v2:
    - none, now after the patch that keeps the port alive for the hangup

 drivers/usb/gadget/function/u_serial.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index 19eea724280b..187a2598181a 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -822,6 +822,19 @@ static void gs_flush_chars(struct tty_struct *tty)
 	spin_unlock_irqrestore(&port->port_lock, flags);
 }
 
+static void gs_flush_buffer(struct tty_struct *tty)
+{
+	struct gs_port	*port = tty->driver_data;
+	unsigned long	flags;
+
+	spin_lock_irqsave(&port->port_lock, flags);
+	kfifo_reset(&port->port_write_buf);
+	spin_unlock_irqrestore(&port->port_lock, flags);
+
+	wake_up_interruptible(&port->drain_wait);
+	tty_wakeup(tty);
+}
+
 static unsigned int gs_write_room(struct tty_struct *tty)
 {
 	struct gs_port	*port = tty->driver_data;
@@ -946,6 +959,7 @@ static const struct tty_operations gs_tty_ops = {
 	.write =		gs_write,
 	.put_char =		gs_put_char,
 	.flush_chars =		gs_flush_chars,
+	.flush_buffer =		gs_flush_buffer,
 	.write_room =		gs_write_room,
 	.chars_in_buffer =	gs_chars_in_buffer,
 	.unthrottle =		gs_unthrottle,
-- 
2.55.0