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Date: Wed, 21 Aug 2013 08:53:51 -0700
From: Peter Grehan <grehan@freebsd.org>
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To: Andre Oppermann <andre@freebsd.org>
Subject: Re: svn commit: r254520 - in head/sys: kern sys
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Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org,
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Hi Andre,

> On 21.08.2013 17:03, Peter Grehan wrote:
>>> The way to go should be 4K clusters as they are native to the
>>> architecture.
>>> IIRC a PCIe DMA can't cross a 4K boundary anyway
>>
>>   That's a 4G boundary, for some devices.
>
> I meant a single PCIe DMA transaction can be at most 4K before it has to
> set up another one?

  4K is the maximum TLP size but that's rarely (ever?) used - 128 or 256 
bytes is more common. A DMA operation is just a sequence of TLPs 
originating from the endpoint.

later,

Peter.