netdev - Re: [RFC v2 1/2] PCI-Express Non-Transparent Bridge Support

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Date:	Wed, 1 Aug 2012 02:18:45 +0000
From:	"Jiang, Dave" <dave.jiang@...el.com>
To:	Jianbin Kang <kjbmail@...il.com>
CC:	"Mason, Jon" <jon.mason@...el.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [RFC v2 1/2] PCI-Express Non-Transparent Bridge Support


On Jul 31, 2012, at 7:10 PM, "Jianbin Kang" <kjbmail@...il.com> wrote:

>> Actually this is what I'm working on now, using async_tx to replace the
>> memcpy.  I believe the changes shouldn't be that significant.
>> 
>> Is the "hardware that can setup dma" you refer to something that does
>> not use this interface?
>> 
> 
> Yes, they use this interface, but split 'memcpy_toio' to two operation:
>     1. setup dma
>     2. wait/poll for the dma to finish.
> So maybe it need to provide a generic function 'tx' for different hardwares.
> 
It's not worth it to do sync DMA. The performance is terrible. 

> If async_tx is available, it's much better than this 'sync dma/memcpy'.
> One problem with async_tx is, it can't detect memcpy error.
> If the remote ntb goes down when async_memcpy is in operation, async_tx
> will trigger an oops.

Yes that is something that needs to be addressed when we get async DMA support working. --
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