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

Jason Wessel
2010-11-05T19:06:01Z

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@windriver.com>
CC: Greg Kroah-Hartman <gregkh@suse.de>
CC: Alan Stern <stern@rowland.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