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Date:   Wed, 20 May 2020 12:50:01 -0400
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Tejas Joglekar <Tejas.Joglekar@synopsys.com>
Cc:     Mathias Nyman <mathias.nyman@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        USB <linux-usb@vger.kernel.org>,
        Mathias Nyman <mathias.nyman@intel.com>,
        John Youn <John.Youn@synopsys.com>
Subject: Re: [RFC PATCH v2 4/4] usb: xhci: Use temporary buffer to
 consolidate SG
Message-ID: <20200520165001.GD11084@rowland.harvard.edu>
References: <cover.1587461220.git.joglekar@synopsys.com>
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On Wed, May 20, 2020 at 06:41:16AM +0000, Tejas Joglekar wrote:
> >> +	urb->transfer_buffer = temp;
> >> +	urb->transfer_dma = dma_map_single(hcd->self.sysdev,
> >> +					   urb->transfer_buffer,
> >> +					   urb->transfer_buffer_length,
> >> +					   dir);
> >> +
> >> +	if (dma_mapping_error(hcd->self.sysdev,
> >> +			      urb->transfer_dma)) {
> >> +		ret = -EAGAIN;
> >> +		kfree(temp);
> >> +	} else {
> >> +		urb->transfer_flags |= URB_DMA_MAP_SINGLE;
> > 
> > Not sure if using URB_DMA_MAP_SINGLE to flag that this urb is boucebuffered is
> > appropriate.
> > 
> > If Greg or Alan don't object then it's fine for me as well.
> > 
> > 
> > 
> @Greg/Alan do you suggest any change for the flag here?

This requires care, because the core will already have set other flags 
for this URB.  I don't remember the details and I don't have time to 
check them now.  But it wouldn't be at all surprising if the core 
doesn't expect an URB both to use SG and to use DMA_MAP_SINGLE.

Alan Stern