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, 2 insertions, 0 deletions

diff --git a/drivers/usb/serial/usb_debug.c b/drivers/usb/serial/usb_debug.c
index 6c9cbb59552a..a9427a8b8672 100644
--- a/drivers/usb/serial/usb_debug.c
+++ b/drivers/usb/serial/usb_debug.c
@@ -15,6 +15,7 @@
 #include <linux/usb.h>
 #include <linux/usb/serial.h>
 
+#define URB_DEBUG_MAX_IN_FLIGHT_URBS    4000
 #define USB_DEBUG_MAX_PACKET_SIZE       8
 
 static struct usb_device_id id_table [] = {
@@ -46,6 +47,7 @@ static struct usb_serial_driver debug_device = {
         .id_table =             id_table,
         .num_ports =            1,
         .open =                 usb_debug_open,
+        .max_in_flight_urbs =   URB_DEBUG_MAX_IN_FLIGHT_URBS,
 };
 
 static int __init debug_init(void)