[RFC PATCH 2/5] usb-serial-console: try to poll the hcd vs dropping data

[Jason Wessel]

This patch tries to solve the problem that printk data is dropped
because there are too many outstanding transmit urb's while trying to
execute printk's to a console.  You can observe this by booting a
kernel with a serial console and cross checking the dmesg output with
what you received on the console or doing something like
"echo t > /proc/sysrq-trigger".

This patch takes the route of forcibly polling the hcd device to drain
the urb queue in order to initiate the bulk write call backs.

A few millisecond penalty will get incurred to allow the hcd
controller to complete a write urb, else the console data is thrown
away.  Even with any kind of delay, the latency is still lower than
using a traditional serial port uart due to the fact that there are
bigger buffer for use with the USB serial console kfifo
implementation.

Signed-off-by: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
CC: Greg Kroah-Hartman <gregkh@xxxxxxx>
CC: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
---
 drivers/usb/serial/console.c |   42 +++++++++++++++++++++++++++---------------
 1 files changed, 27 insertions(+), 15 deletions(-)

diff --git a/drivers/usb/serial/console.c b/drivers/usb/serial/console.c
index 1ee6b2a..b09832f 100644
--- a/drivers/usb/serial/console.c
+++ b/drivers/usb/serial/console.c
@@ -197,13 +197,37 @@ static int usb_console_setup(struct console *co, char *options)
 	return retval;
 }
 
+static void usb_do_console_write(struct usb_serial *serial,
+				 struct usb_serial_port *port,
+				 const char *buf, unsigned count)
+{
+	int retval;
+	int loops = 100;
+try_again:
+	/* pass on to the driver specific version of this function if
+	   it is available */
+	if (serial->type->write)
+		retval = serial->type->write(NULL, port, buf, count);
+	else
+		retval = usb_serial_generic_write(NULL, port, buf, count);
+	if (retval < count && retval >= 0 && loops--) {
+		/* poll the hcd device because the queue is full */
+		count -= retval;
+		buf += retval;
+		udelay(100);
+		usb_poll_irq(serial->dev);
+		usb_poll_irq_schedule_flush();
+		goto try_again;
+	}
+	dbg("%s - return value : %d", __func__, retval);
+}
+
 static void usb_console_write(struct console *co,
 					const char *buf, unsigned count)
 {
 	static struct usbcons_info *info = &usbcons_info;
 	struct usb_serial_port *port = info->port;
 	struct usb_serial *serial;
-	int retval = -ENODEV;
 
 	if (!port || port->serial->dev->state == USB_STATE_NOTATTACHED)
 		return;
@@ -230,23 +254,11 @@ static void usb_console_write(struct console *co,
 				break;
 			}
 		}
-		/* pass on to the driver specific version of this function if
-		   it is available */
-		if (serial->type->write)
-			retval = serial->type->write(NULL, port, buf, i);
-		else
-			retval = usb_serial_generic_write(NULL, port, buf, i);
-		dbg("%s - return value : %d", __func__, retval);
+		usb_do_console_write(serial, port, buf, i);
 		if (lf) {
 			/* append CR after LF */
 			unsigned char cr = 13;
-			if (serial->type->write)
-				retval = serial->type->write(NULL,
-								port, &cr, 1);
-			else
-				retval = usb_serial_generic_write(NULL,
-								port, &cr, 1);
-			dbg("%s - return value : %d", __func__, retval);
+			usb_do_console_write(serial, port, &cr, 1);
 		}
 		buf += i;
 		count -= i;
-- 
1.6.3.3

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