[PATCH] usb: add flow control to u_serial

[Robert Jarzmik]

When using the serial console over USB to download files, and
when the USB bandwidth is pushed to its limits, the barebox UDC
device won't be able to absorb all the traffic.

Modify the u_serial gadget so that if there is not enough room
in buffers, don't push USB requests down the gadget driver, so
that it is saturated, and give it a chance to NAK requests.

The previous behaviour was loosing bytes (as kfifo_put is lossy).
The fixed behavious is lossless (based on USB NAK protocol).

While at it, increase a bit buffer sizes to 8kB to absorb heavy
transfers such as a linux kernel image.

Signed-off-by: Robert Jarzmik <robert.jarzmik at free.fr>
---
 drivers/usb/gadget/u_serial.c |   25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/drivers/usb/gadget/u_serial.c b/drivers/usb/gadget/u_serial.c
index 946b4f2..162439e 100644
--- a/drivers/usb/gadget/u_serial.c
+++ b/drivers/usb/gadget/u_serial.c
@@ -74,9 +74,9 @@
  * next layer of buffering.  For TX that's a circular buffer; for RX
  * consider it a NOP.  A third layer is provided by the TTY code.
  */
-#define QUEUE_SIZE		16
+#define QUEUE_SIZE		128
 #define WRITE_BUF_SIZE		8192		/* TX only */
-
+#define RECV_FIFO_SIZE		(1024 * 8)
 /*
  * The port structure holds info for each port, one for each minor number
  * (and thus for each /dev/ node).
@@ -92,7 +92,7 @@ struct gs_port {
 	u8			port_num;
 
 	struct list_head	read_pool;
-	unsigned		n_read;
+	unsigned		read_nb_queued;
 
 	struct list_head	write_pool;
 
@@ -123,7 +123,9 @@ static unsigned gs_start_rx(struct gs_port *port)
 	struct usb_ep		*out = port->port_usb->out;
 	unsigned		started = 0;
 
-	while (!list_empty(pool)) {
+	while (!list_empty(pool) &&
+	       ((out->maxpacket * (port->read_nb_queued + 1) +
+		kfifo_len(port->recv_fifo)) < RECV_FIFO_SIZE)) {
 		struct usb_request	*req;
 		int			status;
 
@@ -134,6 +136,7 @@ static unsigned gs_start_rx(struct gs_port *port)
 		/* drop lock while we call out; the controller driver
 		 * may need to call us back (e.g. for disconnect)
 		 */
+		port->read_nb_queued++;
 		status = usb_ep_queue(out, req);
 
 		if (status) {
@@ -161,8 +164,9 @@ static void gs_read_complete(struct usb_ep *ep, struct usb_request *req)
 		return;
 
 	kfifo_put(port->recv_fifo, req->buf, req->actual);
-
 	list_add_tail(&req->list, &port->read_pool);
+	port->read_nb_queued--;
+
 	gs_start_rx(port);
 }
 
@@ -276,7 +280,7 @@ static int gs_start_io(struct gs_port *port)
 	}
 
 	/* queue read requests */
-	port->n_read = 0;
+	port->read_nb_queued = 0;
 	started = gs_start_rx(port);
 
 	/* unblock any pending writes into our circular buffer */
@@ -396,6 +400,7 @@ static int serial_tstc(struct console_device *cdev)
 	struct gs_port	*port = container_of(cdev,
 					struct gs_port, cdev);
 
+	gs_start_rx(port);
 	return (kfifo_len(port->recv_fifo) == 0) ? 0 : 1;
 }
 
@@ -412,10 +417,14 @@ static int serial_getc(struct console_device *cdev)
 	while (kfifo_getc(port->recv_fifo, &ch)) {
 		usb_gadget_poll();
 		if (is_timeout(to, 300 * MSECOND))
-			break;
+			goto timeout;
 	}
 
+	gs_start_rx(port);
 	return ch;
+timeout:
+	gs_start_rx(port);
+	return -ETIMEDOUT;
 }
 
 static void serial_flush(struct console_device *cdev)
@@ -461,7 +470,7 @@ int gserial_connect(struct gserial *gser, u8 port_num)
 	 */
 	gser->port_line_coding = port->port_line_coding;
 
-	port->recv_fifo = kfifo_alloc(1024);
+	port->recv_fifo = kfifo_alloc(RECV_FIFO_SIZE);
 
 	/*printf("gserial_connect: start ttyGS%d\n", port->port_num);*/
 	gs_start_io(port);
-- 
1.7.10