linux-kernel - Re: USB transfer_buffer allocations on 64bit systems

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Date:	Fri, 9 Apr 2010 11:15:43 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	Daniel Mack <daniel@...aq.de>, <linux-kernel@...r.kernel.org>,
	Pedro Ribeiro <pedrib@...il.com>, <akpm@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, <alsa-devel@...a-project.org>,
	<linux-usb@...r.kernel.org>
Subject: Re: USB transfer_buffer allocations on 64bit systems

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

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