On Wed, May 20, 2020 at 05:00:50PM +0000, Tejas Joglekar wrote:
> Hi,
> On 5/20/2020 10:20 PM, Alan Stern wrote:
> > 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.
> >
> When this quirk is enabled and it is required to consolidate SG list to
> single buffer no other flags are set for the URB.
I don't believe that. What about URB_DMA_MAP_SG or
URB_DMA_MAP_PAGE? The USB core certainly would have set one of them
already. Not only would you need to clear that flag, you want also need
to undo the mapping.
> Only question here is
> if it is good to use DMA_MAP_SINGLE flag for the consolidated SG list
> into a single temporary buffer.
If you call dma_map_single() then USB_DMA_MAP_SINGLE is the appropriate
flag to sest.
Alan Stern
