On Fri, 9 Apr 2010, Oliver Neukum wrote:
Am Freitag, 9. April 2010 16:41:48 schrieb Alan Stern:
It'll work on x86. On incoherent architectures this violates the cacheline rules for DMA-mapping if you have to bounce.
Not true. Consider: The driver allocates a 16-byte buffer (xbuf) divided up into four sets of four bytes, and sets
urb[i].transfer_buffer_dma = xbuf_dma + 4*i;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 my control.
Alan Stern