[1/2] usb: gadget: udc: core: update usb_ep_queue() documentation

[Alan Stern <stern@rowland.harvard.edu>]

On Mon, 26 Mar 2018, Felipe Balbi wrote:

> Mention that ->complete() should never be called from within
> usb_ep_queue().
> 
> Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
> ---
>  drivers/usb/gadget/udc/core.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/usb/gadget/udc/core.c b/drivers/usb/gadget/udc/core.c
> index 50988b21a21b..842814bc0e4f 100644
> --- a/drivers/usb/gadget/udc/core.c
> +++ b/drivers/usb/gadget/udc/core.c
> @@ -238,6 +238,9 @@ EXPORT_SYMBOL_GPL(usb_ep_free_request);
>   * arranges to poll once per interval, and the gadget driver usually will
>   * have queued some data to transfer at that time.
>   *
> + * Note that @req's ->complete() callback must never be called from
> + * within usb_ep_queue() as that can create deadlock situations.
> + *

I think this is highly questionable.  Certainly it was not David 
Brownell's original intention; his dummy-hcd driver will sometimes 
give back a request from within usb_ep_queue() -- and I believe he 
wrote it that way in order to emulate a feature of his net2280 driver.

In this particular case, the problem is that a driver acquires a 
spinlock in its complete() routine, but then it holds that same 
spinlock while submitting a request.  This is a bug; it should be fixed 
in the driver.  The spinlock should be dropped while the request is 
submitted.  I'm sure there are examples whether other drivers do this.

Alan Stern
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