linux-kernel - Re: [PATCH] usb: dwc3: ep0: fix delayed status is queued too early

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Date:	Thu, 8 May 2014 23:01:33 -0400
From:	Zhuang Jin Can <jin.can.zhuang@...el.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Felipe Balbi <balbi@...com>, USB list <linux-usb@...r.kernel.org>,
	linux-omap@...r.kernel.org,
	Kernel development list <linux-kernel@...r.kernel.org>,
	liping.zhou@...el.com, david.a.cohen@...ux.intel.com
Subject: Re: [PATCH] usb: dwc3: ep0: fix delayed status is queued too early

On Thu, May 08, 2014 at 10:25:46AM -0400, Alan Stern wrote:
> On Thu, 8 May 2014, Zhuang Jin Can wrote:
> 
> > > When the host already timed out the control transfer and started a new 
> > > one.  Here's what I'm talking about:
> > > 
> > > 	Host sends a Set-Configuration request.
> > > 
> > > 	The UDC driver calls the gadget driver's setup function.
> > > 
> > > 	The setup function returns DELAYED_STATUS.
> > > 
> > > 	After a few seconds, the host gets tired of waiting and
> > > 	sends a Get-Descriptor request
> > My understanding is dwc3 will return NYET to host for this
> > Get-Descriptor request transaction, as dwc3 is still in STATUS phase,
> > there's no buffer to receive anything in ep0-out.
> 
> dwc3 _cannot_ return NYET to a SETUP packet.  The USB protocol does not 
> allow it.  A device must always respond to SETUP with ACK.
It true that device can not return NYET to a SETUP packet.
A device must always respond to SETUP with ACK _if_ the SETUP packet is
correctly received. Because there's no buffer prepared in ep0 for dwc3
to receive the SETUP packet, I guess there will be no handshake
returned to host. I can confirm this by doing an experiment tomorrow:)

Jincan
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