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

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Date:	Wed, 16 Apr 2014 20:08:03 +0000
From:	Paul Zimmerman <Paul.Zimmerman@...opsys.com>
To:	sundeep subbaraya <sundeep.lkml@...il.com>,
	"balbi@...com" <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

> From: linux-usb-owner@...r.kernel.org [mailto:linux-usb-owner@...r.kernel.org] On Behalf Of sundeep subbaraya
> Sent: Wednesday, April 16, 2014 3:39 AM
> 
> 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

If this function is called in interrupt context, then you can't use
jiffies for your timeout, since jiffies may not get updated while in
interrupt context.

-- 
Paul