linux-kernel - Re: problems doing direct dma from a pci device to pci-e device

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Message-ID: <4AAA6BE7.8040306@compro.net>
Date:	Fri, 11 Sep 2009 11:25:27 -0400
From:	Mark Hounschell <markh@...pro.net>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	linux-pci@...r.kernel.org, Mark Hounschell <dmarkh@....rr.com>,
	linux-kernel@...r.kernel.org
Subject: Re: problems doing direct dma from a pci device to pci-e device

Alan Cox wrote:
>> I now have an AM3 based DFI DK 790FXB-M3H5 motherboard. This board has 3 regular
>> PCI slots and 3 PCI-E (16x) slots. I also have a PCI-E (x4) version of the VMIC-5565
>> reflective memory  card in one of the PCI-E slots and our gpiohsd card in one of the regular
>> PCI slots. All on the motherboard. No expansion slots being used. However I cannot get
>> data from our gpiohsd into the PCI-E VMIC-5565 cards memory. I can certainly get the data there
>> from a userland buffer, no problem. Just not from one card to the other directly. Oh and when
>> I put the regular PCI version of the VMIC into one of the regular PCI slots everything works
>> as expected. They are then both on the same PCI bus and no bridges are involved though.
> 
> Have you verified with the vendor that such DMA works properly ? There is
> a long history of there being boards where some device to device DMA
> exploded or vanished. The arrival of PCI capture cards doing direct to
> video DMA cleaned the world up (eg the BT848) but I wouldn't be suprised
> if this recurred somewhere since they were popular and nobody really
> noticed as they didn't run such an unusual config.
> 

I have not made an inquiry with the vendor yet. I'm pretty sure I have done 
this with this very card in another mother board. I guess I need to dig that one 
up and verify it before I go much further.  

But it's much like you describe above. The data seems to just vanish. I can't
see any way to use my v-metro to find out where it's going either.

> Also does the board have a true IOMMU in the PCI-E side of the system ?
> It's not a chipset I know.
> 

That I don't know for sure but I thought they all did??

I'll verify that the VMIC card is in fact capable in that other MB before
I do anything else.

Thanks
Mark

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