[alsa-devel] USB transfer_buffer allocations on 64bit systems
oliver at neukum.org
Fri Apr 9 16:50:31 CEST 2010
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.
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