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

["Richard B. Johnson" <RJohnson@Route495Software.com>]

----- Original Message ----- 
From: "Mark Hounschell" <markh@compro.net>
To: "Alan Cox" <alan@lxorguk.ukuu.org.uk>
Cc: <linux-pci@vger.kernel.org>; "Mark Hounschell" <dmarkh@cfl.rr.com>; 
<linux-kernel@vger.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!

Cheers,
Richard B. Johnson
http://Route495Software.com/