usb: gadget: udc: core: Document the relation between usb_ep_queue() and completion callback

[Felipe Balbi <felipe.balbi@linux.intel.com>]

Hi,

Alan Stern <stern@rowland.harvard.edu> writes:
> Improve the kerneldoc for usb_ep_queue() to note explicitly that the
> request's completion routine will be called if and only if the return
> value is 0.  The corresponding fact about usb_submit_urb() for the
> host-side API has long been documented, and this has always been the
> intention for the gadget API.  But until now, documentation seems to
> have been lacking.
>
> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
>
> ---

Greg, I have this queued up for v4.18 already, but if you wanna take it
forward on v4.17 it's fine by me.

Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>

>  drivers/usb/gadget/udc/core.c |    6 ++++++
>  1 file changed, 6 insertions(+)
>
> Index: usb-4.x/drivers/usb/gadget/udc/core.c
> ===================================================================
> --- usb-4.x.orig/drivers/usb/gadget/udc/core.c
> +++ usb-4.x/drivers/usb/gadget/udc/core.c
> @@ -241,6 +241,12 @@ EXPORT_SYMBOL_GPL(usb_ep_free_request);
>   * Returns zero, or a negative error code.  Endpoints that are not enabled
>   * report errors; errors will also be
>   * reported when the usb peripheral is disconnected.
> + *
> + * If and only if @req is successfully queued (the return value is zero),
> + * @req->complete() will be called exactly once, when the Gadget core and
> + * UDC are finished with the request.  When the completion function is called,
> + * control of the request is returned to the device driver which submitted it.
> + * The completion handler may then immediately free or reuse @req.
>   */
>  int usb_ep_queue(struct usb_ep *ep,
>  			       struct usb_request *req, gfp_t gfp_flags)
>
> --
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