[alsa-devel] USB transfer_buffer allocations on 64bit systems

Alan Stern stern at rowland.harvard.edu
Fri Apr 9 16:41:48 CEST 2010

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

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