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

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Message-ID: <20100507102408.GM30801@buzzloop.caiaq.de>
Date:	Fri, 7 May 2010 12:24:08 +0200
From:	Daniel Mack <daniel@...aq.de>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	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>,
	Alan Stern <stern@...land.harvard.edu>,
	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, May 07, 2010 at 11:47:37AM +0200, Clemens Ladisch wrote:
> Daniel Mack wrote:
> > The problem is again (summarized):
> > 
> > On 64bit machines, with 4GB or more, the allocated buffers for USB
> > transfers might be beyond the 32bit boundary. In this case, the IOMMU
> > should take care and install DMA bounce buffer to copy over the buffer
> > before the transfer actually happens. The problem is, however, that this
> > copy mechanism takes place when the URB with its associated buffer is
> > submitted, not when the EHCI will actually do the transfer.
> > 
> > In the particular case of audio drivers, though, the contents of the
> > buffers are likely to change after the submission. What we do here
> > is that we map the audio stream buffers which are used by ALSA to
> > the output URBs, so they're filled asychronously. Once the buffer is
> > actually sent out on the bus, it is believed to contain proper audio
> > date. If it doesn't, that's due to too tight audio timing or other
> > problems. This breaks once buffers are magically bounced in the
> > background.
> 
> At least the audio class and ua101 drivers don't do this and fill the
> buffers before they are submitted.

Gnaa, you're right. I _thought_ my code does it the way I described, but
what I wrote is how I _wanted_ to do it, not how it's currently done. I
have a plan to change this in the future.

So unfortunately, that doesn't explain it either. Sorry for the noise.

Daniel
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