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

[Richard B. Johnson]

----- Original Message ----- 
From: "Mark Hounschell" <markh@xxxxxxxxxx>
To: "Alan Cox" <alan@xxxxxxxxxxxxxxxxxxx>
Cc: <linux-pci@xxxxxxxxxxxxxxx>; "Mark Hounschell" <dmarkh@xxxxxxxxxx>; 
<linux-kernel@xxxxxxxxxxxxxxx>
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/






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