linux-kernel - Re: [alsa-devel] USB transfer_buffer allocations on 64bit systems

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Date:	Fri, 7 May 2010 10:45:21 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Daniel Mack <daniel@...aq.de>
cc:	Oliver Neukum <oneukum@...e.de>,
	Clemens Ladisch <clemens@...isch.de>,
	Takashi Iwai <tiwai@...e.de>, <alsa-devel@...a-project.org>,
	Greg KH <gregkh@...e.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Chris Wright <chrisw@...s-sol.org>,
	<iommu@...ts.linux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Ribeiro <pedrib@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [alsa-devel] USB transfer_buffer allocations on 64bit systems

On Fri, 7 May 2010, Daniel Mack wrote:

> > Or to be precise it could be done with coherent memory, but you'd risk
> > transfering partially updated buffers, as you cannot know when DMA will
> > be done and we don't guarantee that DMA will be done right as we transfer.
> 
> If the memory is coherent, it should be possible. And if buffers are
> only partially updated, the audio buffer scheduling is too tight. IOW,
> the configured buffer size of audio clients in userspace is too small.

Even with coherent memory, this dynamic updating of buffer contents 
isn't a good idea.  There's no guarantee that coherent memory won't be 
implemented using a bounce buffer during URB submission.

Alan Stern

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