Re: [RFC PATCH 1/5] usb-hcd: implement polling a specific usb device

Alan Stern
2010-11-06T18:32:38Z

On Fri, 5 Nov 2010, Jason Wessel wrote:

> This patch adds a generic capability to poll a specific usb device and
> run its completion handler.
>
> There will be two users of this functionality.
> 1) usb serial port console devices
> 2) usb keyboards for use with kdb
>
> Any time the usb_irq_poll() function is called it has the possibility
> to queue some urbs for completion at a later time. Any code that uses
> the usb_irq_poll() function must call usb_poll_irq_schedule_flush()
> before all the other usb devices will start working again.

...

> --- a/drivers/usb/core/hcd.c
> +++ b/drivers/usb/core/hcd.c

> @@ -1562,6 +1567,17 @@ void usb_hcd_giveback_urb(struct usb_hcd *hcd, struct urb *urb, int status)
>
> /* pass ownership to the completion handler */
> urb->status = status;
> + if (unlikely(usb_poll_hcd_device)) {
> + /*
> + * Any other device than the one being polled should get
> + * queued for a later flush.
> + */
> + if (usb_poll_hcd_device != urb->dev) {
> + INIT_LIST_HEAD(&urb->poll_list);

Why initialize the list_head here if the next line is going to
overwrite it?

> + list_add(&urb->poll_list, &delayed_usb_complete_queue);

You need to use list_add_tail, not list_add. This is supposed to be a
queue, not a LIFO stack.

> + return;
> + }
> + }
> urb->complete (urb);
> atomic_dec (&urb->use_count);
> if (unlikely(atomic_read(&urb->reject)))
> @@ -2425,6 +2441,47 @@ usb_hcd_platform_shutdown(struct platform_device* dev)
> }
> EXPORT_SYMBOL_GPL(usb_hcd_platform_shutdown);
>
> +void usb_poll_irq(struct usb_device *udev)
> +{

Please add a little kerneldoc explaining what this function does and
why it is here. Also mention that it must be called with interrupts
disabled.

> + struct usb_hcd *hcd;
> +
> + hcd = bus_to_hcd(udev->bus);
> + usb_poll_hcd_device = udev;
> + usb_hcd_irq(0, hcd);
> + usb_poll_hcd_device = NULL;
> +}
> +EXPORT_SYMBOL_GPL(usb_poll_irq);
> +
> +static void usb_poll_irq_flush_helper(struct work_struct *dummy)
> +{
> + struct urb *urb;
> + struct urb *scratch;
> + unsigned long flags;
> +
> + mutex_lock(&usb_poll_irq_flush_mutex);

What is this mutex used for?

> + local_irq_save(flags);

Isn't this function meant to be called with interrupts disabled on all
CPUs anyway?

> + list_for_each_entry_safe(urb, scratch, &delayed_usb_complete_queue,
> + poll_list) {
> + list_del(&urb->poll_list);
> + urb->complete(urb);
> + atomic_dec(&urb->use_count);
> + if (unlikely(atomic_read(&urb->reject)))
> + wake_up(&usb_kill_urb_queue);
> + usb_put_urb(urb);
> + }
> + local_irq_restore(flags);
> + mutex_unlock(&usb_poll_irq_flush_mutex);
> +}
> +
> +static DECLARE_WORK(usb_poll_irq_flush_work, usb_poll_irq_flush_helper);
> +
> +
> +void usb_poll_irq_schedule_flush(void)
> +{
> + schedule_work(&usb_poll_irq_flush_work);
> +}
> +EXPORT_SYMBOL_GPL(usb_poll_irq_schedule_flush);

Why do you want to do this in a workqueue? Just move all the code from
usb_poll_irq_flush_helper over here directly.

> --- a/include/linux/usb.h
> +++ b/include/linux/usb.h

> @@ -1188,6 +1192,7 @@ struct urb {
> struct list_head urb_list; /* list head for use by the urb's
> * current owner */
> struct list_head anchor_list; /* the URB may be anchored */
> + struct list_head poll_list; /* Added to the poll queue */
> struct usb_anchor *anchor;
> struct usb_device *dev; /* (in) pointer to associated device */
> struct usb_host_endpoint *ep; /* (internal) pointer to endpoint */

You don't need to add an additional list_head to struct urb -- it
already contains too much stuff. Simply use urb_list; that's what it's
for.

Alan Stern