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

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Message-ID: <1273483265.372.3383.camel@macbook.infradead.org>
Date:	Mon, 10 May 2010 10:21:05 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	stern@...land.harvard.edu, daniel@...aq.de, clemens@...isch.de,
	tiwai@...e.de, alsa-devel@...a-project.org, gregkh@...e.de,
	konrad.wilk@...cle.com, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, chrisw@...s-sol.org,
	iommu@...ts.linux-foundation.org, andi@...stfloor.org,
	pedrib@...il.com, akpm@...ux-foundation.org
Subject: Re: [alsa-devel] USB transfer_buffer allocations on 64bit systems

On Mon, 2010-05-10 at 11:50 +0900, FUJITA Tomonori wrote:
> On Fri, 7 May 2010 10:51:10 -0400 (EDT)
> Alan Stern <stern@...land.harvard.edu> wrote:
> 
> > On Fri, 7 May 2010, Daniel Mack wrote:
> > 
> > > > 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.
> > 
> > At one point we tried an experiment, printing out the buffer and DMA 
> > addresses.  I don't recall seeing anything obviously wrong, but if an 
> > IOMMU was in use then that might not mean anything.  Is it possible 
> > that the IOMMU mappings sometimes get messed up for addresses above 4 
> > GB?
> 
> You mean that an IOMMU could allocate an address above 4GB wrongly? If
> so, IIRC, all the IOMMU implementations use dev->dma_mask and
> dev->coherent_dma_mask properly. And the DMA address space of the
> majority of IOMMUs are limited less than 4GB.

The Intel IOMMU code will use dev->dma_mask and dev->coherent_dma_mask
properly. It is not limited to 4GiB, but it will tend to give virtual
DMA addresses below 4GiB even when a device is capable of more; it'll
only give out higher addresses when the address space below 4GiB is
exhausted.

-- 
dwmw2

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