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

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Message-ID: <20100409202533.GA8983@phenom.dumpdata.com>
Date:	Fri, 9 Apr 2010 16:25:33 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Pedro Ribeiro <pedrib@...il.com>, Daniel Mack <daniel@...aq.de>,
	Robert Hancock <hancockrwd@...il.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Greg KH <gregkh@...e.de>, alsa-devel@...a-project.org,
	linux-usb@...r.kernel.org
Subject: Re: [LKML] Re: USB transfer_buffer allocations on 64bit systems

On Fri, Apr 09, 2010 at 03:34:06PM -0400, Alan Stern wrote:
> On Fri, 9 Apr 2010, Pedro Ribeiro wrote:
> 
> > > The DMA pointers do indeed look sane. I wanted to take a deeper look at
> > > this and set up a 64bit system today. However, I fail to see the problem
> > > here. Pedro, how much RAM does your machine have installed?
> 
> > It has 4 GB.
> 
> That means DMA mapping cannot be the cause of the problem.  :-(

That isn't entirely true. The BIOS usually allocates a 256 MB ACPI/PCI hole
that is under the 4GB.

So end up with 3.7 GB, then the 256MB hole, and then right above the 4GB
you the the remaining memory: 4.3GB.

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