On Fri, 9 Apr 2010, Oliver Neukum wrote:
Am Freitag, 9. April 2010 00:20:36 schrieb Alan Stern:
That would work, but it doesn't match the way existing drivers use the interface. For example, the audio driver allocates a 16-byte coherent buffer and then uses four bytes from it for each of four different URBs.
That will not work with any fallback that does not yield a coherent buffer.
What you mean isn't entirely clear. But it certainly does work in various circumstances that don't yield coherent buffers. For example, it works if the controller uses PIO instead of DMA. It also works if the controller uses DMA and the URBs have to be bounced.
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.
So it seems to me that if you want to share a buffer between URBs, it must be coherent.
No. But it must be allocated via usb_alloc_buffer() (or whatever that routine gets renamed to).
Alan Stern