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

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Message-Id: <201004091650.31488.oliver@neukum.org>
Date:	Fri, 9 Apr 2010 16:50:31 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Alan Stern <stern@...land.harvard.edu>
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

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.

	Regards
		Oliver
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