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    SubjectRe: [PATCH 4/5] usb-hcd,usb-console: poll hcd device to force usb console writes
    On Tue, 16 Mar 2010, Jason Wessel wrote:

    > This patch tries to solve the problem that data is lost because there
    > are too many outstanding transmit urb's while trying to execute
    > printk's to a console. The same is true if you try 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. This
    > only happens if the device is a usb serial console device that sets
    > the max_in_flight_urbs to a non zero value in the serial device
    > structure.
    > A few millisecond penalty will get incurred to allow the hcd controller
    > to complete a write urb, else the console data is thrown away.
    > The max_in_flight_urbs was reduced in the usb_debug driver because it
    > is highly desired to push things out to the console in a timely
    > fashion and there is no need to have a queue that large for the
    > interrupt driven mode of operation when used through the tty
    > interface.

    > diff --git a/include/linux/usb.h b/include/linux/usb.h
    > index 8c9f053..a7d6cf7 100644
    > --- a/include/linux/usb.h
    > +++ b/include/linux/usb.h
    > @@ -569,6 +569,9 @@ static inline void usb_mark_last_busy(struct usb_device *udev)
    > /*-------------------------------------------------------------------------*/
    > +/* for polling the hcd device */
    > +extern void usb_poll_irq(struct usb_device *udev);
    > +

    The points Johan Hovold made about max_in_flight_urbs are well taken
    and they should be addressed before this patch is accepted.

    The parts about usb_poll_irq() are fine, although the comment above
    should be changed to something more like:

    /* for polling the device's host controller */

    since "hcd" is short for "host controller driver", and "polling the
    host controller driver device" doesn't make much sense. The same goes
    for the comment in usb_do_console_write().

    Alan Stern

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