linux-kernel - Re: extra large DMA buffer for PCI-E device under UIO

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Message-ID: <20111122172725.GK4387@parisc-linux.org>
Date:	Tue, 22 Nov 2011 10:27:25 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Jean-Francois Dagenais <jeff.dagenais@...il.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	"Hans J. Koch" <hjk@...sjkoch.de>, Greg KH <gregkh@...e.de>,
	tglx@...utronix.de, linux-pci@...r.kernel.org,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: extra large DMA buffer for PCI-E device under UIO

On Tue, Nov 22, 2011 at 11:54:22AM -0500, Jean-Francois Dagenais wrote:
> That is quite interesting. It really seems like my VT-d recipe to create 128MB for my PCI-e
> FPGA to write into is covered by this patch.
> 
> My problem is that our FPGA is connected to one of the atom E6XX's PCI-e links, so no
> iommu :( Since our first product had VT-d, the FPGA, uio based module and userspace
> code is designed such that the device sees a huge contiguous memory chunk. This is key
> to the performance of the FPGA, which is essentially decoupled from the CPU for it's real-time
> acquisition.

Is it really key?  If you supported, ohidon'tknow, 2MB pages, you'd
need 64 entries in the FPGA to store the addresses of those 2MB pages,
which doesn't sound like a huge burden.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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