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

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Message-ID: <CALHRZur7+MaYXbWOfbvjggPN7_eYAE+th5VNbcLP=ehvm0HGsA@mail.gmail.com>
Date:	Tue, 22 Apr 2014 12:58:41 +0530
From:	sundeep subbaraya <sundeep.lkml@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	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 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.
>
> 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.

Thanks Alan and Felipe,
Sundeep

>
> HTH,
>
> Alan Stern
>
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