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

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Date:	Fri, 9 Apr 2010 08:04:35 +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 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. So it seems to me that
if you want to share a buffer between URBs, it must be coherent.

	Regards
		Oliver
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