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
signature.asc
Description: Digital signature
