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

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Date:	Tue, 3 Jan 2012 15:33:57 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	"ustc.mail" <backyes@...l.ustc.edu.cn>
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, 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?

Is the output/input port on a seperate device function? Or is it
just a specific MMIO BAR in your PCI device?

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