[alsa-devel] USB transfer_buffer allocations on 64bit systems

[Oliver Neukum]

Am Freitag, 9. April 2010 17:15:43 schrieb Alan Stern:
> > > Then usb_submit_urb(urb[i]) will copy the appropriate four bytes to a
> > > bounce buffer and map the bounce buffer.  Accesses to the other parts
> > > of xbuf won't violate the cacheline rules, because xbuf isn't mapped
> > > for DMA -- only the bounce buffer is.  When urb[i] completes, the
> > > bounce buffer contents will be copied back to the original four bytes
> > > in xbuf.  Again, there is no violation of cacheline rules.
> > 
> > I think you are assuming that either every or no part of the buffer is mapped
> > for DMA in place. I don't think you can assume that.
> 
> Yes I can, because the code that makes this decision is part of 
> usbcore and it is under m

It seems to me that in usbcore you can positively know that a buffer
will be mapped. However if the mapping is not done in usbcore you
cannot know what the HCD driver will do to a buffer, in particular
you don't know whether it will be processed by PIO or mapped for
DMA.

Maybe I understand this wrongly. Which code exactly were you refering to?

	Regards
		Oliver