On Fri, 9 Apr 2010, Oliver Neukum wrote:
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.
The mapping is always done either by usb_buffer_alloc() or by map_urb_for_dma(). Both functions are in usbcore.
Maybe I understand this wrongly. Which code exactly were you refering to?
Search for usages of "syncbuf" and "sync_dma" in sound/usb/usbaudio.c.
Alan Stern