On Monday, September 12, 2016 9:09:16 AM CEST Alan Stern wrote:
> > diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
> > index 0406a59..66364ea 100644
> > --- a/drivers/usb/core/message.c
> > +++ b/drivers/usb/core/message.c
> > @@ -1863,6 +1863,12 @@ free_interfaces:
> > intf->dev.type = &usb_if_device_type;
> > intf->dev.groups = usb_interface_groups;
> > intf->dev.dma_mask = dev->dev.dma_mask;
> > + /* Propagate dma_pfn_offset to USB interface.
> > + * This is especially required by mass storage interface
> > + * which relies on SCSI layer and
> > scsi_calculate_bounce_limit()
> > + * to set the bounce buffer limit based on dma_pfn_offset.
> > + */
> > + intf->dev.dma_pfn_offset = dev->dev.dma_pfn_offset;
>
> I really think this comment isn't necessary. It seems pretty obvious
> that if you're copying DMA-related fields from one device to another
> then you'll want to copy the dma_pfn_offset along with everything else.
> No?
>
> The explanation in the Changelog is enough, IMO.
I asked for a code comment here as what we are doing here is a bit
fishy, but the comment (evidently) doesn't quite capture it. I would
add (above the dma_mask assignment) something like
/*
* Fake a dma_mask/offset for the USB device:
* We cannot really use the dma-mapping API (dma_alloc_* and
* dma_map_*) for USB devices but instead need to use
* usb_alloc_coherent and pass data in 'urb's, but some subsystems
* manually look into the mask/offset pair to determine whether
* they need bounce buffers.
* Note: calling dma_set_mask() on a USB device would set the
* mask for the entire HCD, so don't do that.
*/
Arnd
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