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

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Date:	Wed, 30 Apr 2014 11:54:37 -0500
From:	Felipe Balbi <balbi@...com>
To:	sundeep subbaraya <sundeep.lkml@...il.com>
CC:	"balbi@...com" <balbi@...com>,
	Alan Stern <stern@...land.harvard.edu>,
	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 Wed, Apr 23, 2014 at 09:57:52AM +0530, sundeep subbaraya wrote:
> >> > 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 ?
> 
> No. IP does/need not know that it has to wait for Y bytes.We just
> write X bytes into
> HW buffer and count as X in buffer count register. IP generates CRC
> for bytes based
> on Count register and sends data to Host. Let us consider this
> scenario of bulk IN transfer:
> req.length = 5120 and   wMaxPacketSize = 512, ep_queue is called once
> and is returned with
> status 0. In ep_queue this code snippet,
>                if (xudc_write_fifo(ep, req) == 1)
>                         req = NULL;
>                if(req != NULL)
>                          list_add_tail(&req->queue, &ep->queue);
> 
> xudc_write_fifo does the following if HW buffers not busy:
>      copies 512 bytes to HW buffer
>      set count and ready registers so that IP can start data transfer to host
>      changes req.actual to 512 and returns 0(if req.length >
> wMaxPacketSize) and 1(if req.length < wMaxPacketSize).

you should return a proper error code, not 1.

> Since return is zero this request is added to queue. When data
> transfer to host is completed IP generates
> an interrupt. In the interrupt handler we again call write_fifo if
> request list is not empty.
>        if (list_empty(&ep->queue))
>                 req = NULL;
>         else
>                 req = list_entry(ep->queue.next, struct xusb_req, queue);
>         if (!req)
>                 return;

okay, this can be improved a bit though:

	if (list_empty(&ep->queue))
		return;

	req = list_first_entry(&ep->queue, struct xusb_req, queue);

>         if (ep->is_in)
>                 xudc_write_fifo(ep, req);
>         else
>                 xudc_read_fifo(ep, req);
> 
> This happens 10 times(since length 5120) and completion is called.

ok.

> > If there's no space in the FIFO yet, why copy data at all ?
> 
> If HW buffers are busy(IP is still transferring previous data to Host
> from buffer) then xudc_write_fifo returns 0 without changing
> req.actual. When previous data transfer completes then Interrupt then
> again write_fifo from handler.

ok, sounds good to me.

> >> > Thus there never is any need for the gadget driver to queue the request
> >> > again.
> 
> Yes
> 
> >> >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.
> 
> Hope this helps.

it does, but please sort out the small comments I had above.

cheers

-- 
balbi

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