linux-kernel - Re: 【Question】Whether it's legal to enable same physical DMA memory mapped for different NIC device?

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Date:	Wed, 4 Jan 2012 10:44:11 +0800
From:	Yanfei Wang <backyes@...l.ustc.edu.cn>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	linux-pci@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: 【Question】Whether it's legal to enable same physical DMA memory mapped for different NIC device?

On Wed, Jan 4, 2012 at 4:33 AM, Konrad Rzeszutek Wilk
<konrad.wilk@...cle.com> wrote:
> On Wed, Dec 07, 2011 at 10:16:40PM +0800, ustc.mail wrote:
>> Dear all,
>>
>> In NIC driver, to eliminate the  overhead of dma_map_single() for DMA
>> packet data,  we have statically allocated huge DMA memory buffer ring
>> at once instead of calling dma_map_single() per packet.  Considering
>> to further reduce the copy overhead between different NIC(port) ring
>> while forwarding, one packet from a input NIC(port) will be
>> transferred to output NIC(port) with no any copy action.
>>
>> To satisfy this requirement, the packet memory should be mapped into
>> input port and unmapped when leaving input port, then mapped into
>> output port and unmapped later.
>>
>> Whether it's legal to map the same DMA memory into input and output
>> port simultaneously? If it's not, then the zero-copy for packet
>> forwarding is not feasible?
>>
>
> Did you ever a get a response about this?
No.
>
> Is the output/input port on a seperate device function? Or is it
> just a specific MMIO BAR in your PCI device?
>
Platform: x86, intel nehalem 8Core NUMA, linux 2.6.39, 10G
82599NIC(two ports per NIC card);
Function: Forwarding packets between different ports.
Targets: Forwarding packets with Zero-Overhead, despite other obstacles.

--Yanfei
>> Hope PCI expert to post your suggestion.
>>
>> Thanks!
>>
>> --Yanfei
>>
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