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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
