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

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Message-ID: <CALHRZuq6=TMryHtbjTw7rYoGwbu2w0BTdxiOA95HYS3_CG6vwQ@mail.gmail.com>
Date:	Thu, 17 Apr 2014 15:01:37 +0530
From:	sundeep subbaraya <sundeep.lkml@...il.com>
To:	balbi@...com
Cc:	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 Felipe,

On Wed, Apr 16, 2014 at 11:32 PM, Felipe Balbi <balbi@...com> wrote:
> Hi,
>
> On Wed, Apr 16, 2014 at 04:09:28PM +0530, sundeep subbaraya wrote:
>> Hi Felipe,
>>
>> On Thu, Apr 3, 2014 at 8:28 PM, Felipe Balbi <balbi@...com> wrote:
>>
>> >> +static int start_dma(struct xusb_ep *ep, u32 src, u32 dst, u32 length)
>> >
>> > please prepend this with xudc_, it makes tracing a lot easier.
>> >
>> >> +{
>> >> +     struct xusb_udc *udc;
>> >> +     int rc = 0;
>> >> +     unsigned long timeout;
>> >> +
>> >> +     udc = ep->udc;
>> >> +     /*
>> >> +      * Set the addresses in the DMA source and
>> >> +      * destination registers and then set the length
>> >> +      * into the DMA length register.
>> >> +      */
>> >> +     udc->write_fn(udc->base_address, XUSB_DMA_DSAR_ADDR_OFFSET, src);
>> >> +     udc->write_fn(udc->base_address, XUSB_DMA_DDAR_ADDR_OFFSET, dst);
>> >> +     udc->write_fn(udc->base_address, XUSB_DMA_LENGTH_OFFSET, length);
>> >> +
>> >> +     /*
>> >> +      * Wait till DMA transaction is complete and
>> >> +      * check whether the DMA transaction was
>> >> +      * successful.
>> >> +     */
>> >> +     while ((udc->read_fn(ep->udc->base_address + XUSB_DMA_STATUS_OFFSET) &
>> >> +                     XUSB_DMA_DMASR_BUSY) == XUSB_DMA_DMASR_BUSY) {
>> >> +             timeout = jiffies + 10000;
>> >> +
>> >> +             if (time_after(jiffies, timeout)) {
>> >> +                     rc = -ETIMEDOUT;
>> >> +                     goto clean;
>> >> +             }
>> >> +     }
>> >
>> > don't you get an IRQ for DMA completion ? If you do, you could be using
>> > wait_for_completion()
>>
>> This function is called in interrupt context when buffer is ready or
>> free. It initiates DMA to transfer data from IP buffer to memory.
>> Hence it waits in busy loop till DMA completes
>
> wait, so you start_dma() before your gadget driver asks you to ?

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.
when buffer processed interrupt occurs, handler starts dma if there is
request in queue and calls complete call back (when actual == length)
Hence answer is Yes for some transfers (start dma called from
interrupt handler not from ep_queue).

Thanks,
Sundeep.B.S.
>
> --
> balbi
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