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

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Date:	Wed, 7 Apr 2010 22:10:17 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Robert Hancock <hancockrwd@...il.com>
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

On Wed, 7 Apr 2010, Robert Hancock wrote:

> >> The problem is appearantly the way the transfer buffer is allocated in
> >> the drivers. In the snd-usb-caiaq driver, I used kzalloc() to get memory
> >> which works fine on 32bit systems. On x86_64, however, it seems that
> >> kzalloc() hands out memory beyond the 32bit addressable boundary, which
> >> the DMA controller of the 32bit PCI-connected EHCI controller is unable
> >> to write to or read from. Am I correct on this conclusion?
> >
> > That seems like the right answer.  You are correct that an EHCI
> > controller capable only of 32-bit memory accesses would not be able to
> > use a buffer above the 4 GB line.

> AFAIK, the driver shouldn't have to worry about this at all. When the 
> buffer gets DMA-mapped for the controller, the DMA mapping code should 
> see that the device has a 32-bit DMA mask and either bounce or IOMMU-map 
> the memory so that it appears below 4GB.

That's true.  It would of course be more efficient for the buffer to be
allocated below 4 GB, but it should work okay either way.  Daniel, do
you have any idea why it fails?

Alan Stern

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