linux-kernel - Re: [PATCH v2 2/2] usb: gadget: Add xilinx axi usb2 device support

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Message-ID: <20140422144944.GF5524@saruman.home>
Date:	Tue, 22 Apr 2014 09:49:44 -0500
From:	Felipe Balbi <balbi@...com>
To:	sundeep subbaraya <sundeep.lkml@...il.com>
CC:	Alan Stern <stern@...land.harvard.edu>,
	Felipe Balbi <balbi@...com>,
	Subbaraya Sundeep Bhatta <subbaraya.sundeep.bhatta@...inx.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Michal Simek <michals@...inx.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Subbaraya Sundeep Bhatta <sbhatta@...inx.com>
Subject: Re: [PATCH v2 2/2] usb: gadget: Add xilinx axi usb2 device support

Hi,

On Tue, Apr 22, 2014 at 12:58:41PM +0530, sundeep subbaraya wrote:
> Hi,
> 
> On Mon, Apr 21, 2014 at 10:09 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> > On Mon, 21 Apr 2014, Felipe Balbi wrote:
> >
> >> Hi,
> >>
> >> On Fri, Apr 18, 2014 at 07:34:08PM +0530, sundeep subbaraya wrote:
> >>
> >> <snip>
> >>
> >> > >> in ep_queue driver starts dma transfer from/to IP buffer to/from req->buf.
> >> > >> If transfer is completed then request is not added to ep request queue
> >> > >> and returns from ep_queue.
> >> > >> If transfer is not completed (actual < length) then request is added
> >> > >> to queue and returns from ep_queue.
> >> > >
> >> > > This is wrong. Why wouldn't you give gadget driver the chance to decide
> >> > > if it needs to queue the request again or not ?
> >> >
> >> > When does gadget driver decides to queue the same request again?
> >> > if -EBUSY is returned from ep_queue or req.status != 0 in completion
> >> > routine?
> >>
> >> whenever it so decides. Different gadget drivers might have different
> >> requirements. The code is open and sits under drivers/usb/gadget/ why
> >> don't you have a read ?
> >
> > I get the impression that the two of you are arguing past each other.
> > It appears that Sundeep is talking about transferring data from the
> > gadget driver's buffer to an internal buffer in the UDC hardware, but
> > Felipe is talking about transferring data from the UDC to the host.
> >
> > As I understand it, Sundeep said that when the gadget driver queues a
> > data-IN request, the UDC driver copies as much of the data buffer as
> > possible into a hardware FIFO.  If it succeeds in copying all the data
> > into the FIFO then the request's completion routine gets called
> > immediately, even though the data doesn't get sent from the FIFO to the
> > host until the host asks for it.
> >
> > If only part of the data can be copied into the FIFO then the request
> > is added to the ep's request queue before the usb_ep_queue() call
> > returns.  When space becomes available in the FIFO, the data will be
> > copied and eventually sent to the host.  When all the data has been
> > copied to the FIFO, the request's completion routine will be called.

there seems to be a slight problem with this approach: how will the IP
know that even though you copied X bytes into the FIFO, it should wait
for another Y bytes before shifting data to the wire ? How will it know
that it shouldn't generate CRC yet because there's still data to be
added ?

If there's no space in the FIFO yet, why copy data at all ?

> > Thus there never is any need for the gadget driver to queue the request
> > again.  An incomplete transfer means the FIFO didn't have enough room
> > when the request was submitted; it doesn't mean that the data didn't
> > eventually get sent to the host.
> 
> Exactly Alan,this is what I was trying to say. Probably I was not
> clear in explaining. I didnt see any harm this way and even this
> implementation is same like at91_udc.c. I have been reading
> mas_storage to understand when does gadget driver tries to enqueue a
> request again. Since different gadget drivers might have different
> requirements (agree with Felipe), wanted to know criteria for queuing
> a same request again.
> 
> I will change this implementation as per Felipe comments and test with
> some of the gadgets.

Let's see, please help me understand the questions above.

-- 
balbi

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