USB: usb_debug, usb_generic_serial: implement multi urb write

The usb_debug driver, when used as the console, will always fail to
insert the carriage return and new line sequence as well as randomly
drop console output.  This is a result of only having the single
write_urb and that the tty layer will have a lock that prevents the
processing of the back to back urb requests.

The solution is to allow more than one urb to be outstanding and have
a slightly deeper transmit queue.  The idea and some code is borrowed
from the ftdi_sio usb driver.

The generic usb serial driver was modified so as to allow the classic
method of 1 write urb, or a multi write urb scheme with N allowed
outstanding urbs where N is controlled by max_in_flight_urbs.  When
max_in_flight_urbs in a "struct usb_serial_driver" is non zero the
multi write urb scheme will be used.

The size of 4000 was selected for the usb_debug driver so that the
driver lowers possibility of losing the queued console messages during
the kernel startup.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/usb/serial.h b/include/linux/usb/serial.h
index 8cdfed738fe4..e2938fd179e2 100644
--- a/include/linux/usb/serial.h
+++ b/include/linux/usb/serial.h
@@ -91,6 +91,9 @@ struct usb_serial_port {
 	int			write_urb_busy;
 	__u8			bulk_out_endpointAddress;
 
+	int			tx_bytes_flight;
+	int			urbs_in_flight;
+
 	wait_queue_head_t	write_wait;
 	struct work_struct	work;
 	char			throttled;
@@ -207,6 +210,7 @@ struct usb_serial_driver {
 	struct device_driver	driver;
 	struct usb_driver	*usb_driver;
 	struct usb_dynids	dynids;
+	int			max_in_flight_urbs;
 
 	int (*probe)(struct usb_serial *serial, const struct usb_device_id *id);
 	int (*attach)(struct usb_serial *serial);