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

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Date:	Fri,  5 Nov 2010 14:05:03 -0500
From:	Jason Wessel <jason.wessel@...driver.com>
To:	stern@...land.harvard.edu
Cc:	gregkh@...e.de, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
	Jason Wessel <jason.wessel@...driver.com>
Subject: [RFC PATCH 2/5] usb-serial-console: try to poll the hcd vs dropping data

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@...driver.com>
CC: Greg Kroah-Hartman <gregkh@...e.de>
CC: Alan Stern <stern@...land.harvard.edu>
---
 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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