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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