Re: [RFC PATCH v2 4/4] usb: xhci: Use temporary buffer to consolidate SG

[Alan Stern <stern@rowland.harvard.edu>]

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