Recently, I test my usb host driver(not ehci or any widely used usb
host) with a Ralink's wifi device(use a linux-3.0.39 kernel).
But, if transferring data through FTP for a long time,  I found that
usb_hcd_link_urb_to_ep() (It is called by hcd's urb_enqueue function)
returns -EPERM sometimes.

After digging Ralink's wifi driver, I found, it seems that my host
driver some time try to link a killing urb to ep.
(The calling path of usb_kill_urb() is:
rt2x00usb_watchdog()->rt2x00usb_dma_timeout()->rt2x00queue_dma_timeout()->rt2x00usb_watchdog_tx_dma()->rt2x00queue_flush_queue()->rt2x00usb_flush_queue()->rt2x00queue_for_each_entry()->rt2x00usb_flush_entry()->usb_kill_urb()
)

What I hope to know is: it a right result? or it seems there is a bug
in my driver?or because my driver's speed is not high enough(It seems
that it happends more frequently in a low speed)?
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