linux-kernel - Re: [PATCH v5 4/6] usb: gadget: add mechanism to specify an explicit status stage

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Message-ID: <Pine.LNX.4.44L0.1901181148100.1425-100000@iolanthe.rowland.org>
Date:   Fri, 18 Jan 2019 11:52:57 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Paul Elder <paul.elder@...asonboard.com>
cc:     laurent.pinchart@...asonboard.com,
        <kieran.bingham@...asonboard.com>, <b-liu@...com>, <rogerq@...com>,
        <balbi@...nel.org>, <gregkh@...uxfoundation.org>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 4/6] usb: gadget: add mechanism to specify an explicit
 status stage

On Fri, 18 Jan 2019, Paul Elder wrote:

> > > I meant the functions (procedures) in the function driver, so the setup
> > > handler (uvc_function_setup), the completion handler
> > > (uvc_function_ep0_complete), and the status sender (uvc_send_response),
> > > although the last one actually sends the data stage for control IN.
> > > So after the status is sent on the uvc gadget driver's end, its
> > > completion handler is called again without the setup handler being
> > > called beforehand and I cant figure out why.
> > 
> > Isn't this what you should expect?  Every usb_request, if it is queued
> > successfully, eventually gets a completion callback.  That promise is
> > made by every UDC driver; it's part of the gadget API.  So for a
> > control transfer with a data stage, you expect to have:
> > 
> > 	Setup handler called
> > 	Data-stage request submitted
> > 	Data-stage request completion callback
> > 	Status-stage request submitted
> > 	Status-stage request completion callback
> > 
> > Thus, two completion callbacks but only one setup callback.
> 
> omg how did I not notice this :/
> 
> I guess I have to fix the uvc function driver so it works with that.
> musb doesn't call the status stage completion callback though; not that
> it does anything so it seems fine to me, but indeed the function driver
> has to be ready for it if it is called.

musb _has_ to call the status-stage completion callback.  As just one
reason, if the explicit_status flag isn't set then that callback is
responsible for deallocating the status request.  Without it, the
status request will leak.

Alan Stern