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

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Message-ID: <4AAAA814.3000706@compro.net>
Date:	Fri, 11 Sep 2009 15:42:12 -0400
From:	Mark Hounschell <markh@...pro.net>
To:	"Richard B. Johnson" <RJohnson@...te495Software.com>
CC:	Mark Hounschell <dmarkh@....rr.com>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: problems doing direct dma from a pci device to pci-e device

Richard B. Johnson wrote:
> ----- Original Message ----- 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>
> Sent: Friday, September 11, 2009 11:25 AM
> 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.
> 
> The read I mentioned was a read immediately following the DMA operation.
> Some PCI (PCI-X using
> a HyperTransport serial bus) implimentations deliberately time-out to
> prevent a hung system. The result
> that I've seen was no data anywhere. It just vanished!
> 
> Reading the destination a few milliseconds (or seconds) later won't
> prevent this timeout!
> 
 
How about a couple hundred usecs? Along that line, I can make it do dma reads
from that VMIC card also. All I get is ones. It's strange that in this case, my gpiohsd
isn't reporting any kind of pci bus error in it's status register.

Thanks
Mark


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