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Re: [Qemu-devel] [RFC] QOMification of AXI stream

From: Edgar E. Iglesias
Subject: Re: [Qemu-devel] [RFC] QOMification of AXI stream
Date: Mon, 11 Jun 2012 17:56:28 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Mon, Jun 11, 2012 at 10:03:56AM -0500, Anthony Liguori wrote:
> On 06/11/2012 09:53 AM, Peter Maydell wrote:
> >On 11 June 2012 15:38, Edgar E. Iglesias<address@hidden>  wrote:
> >>On Mon, Jun 11, 2012 at 02:39:56PM +0100, Peter Maydell wrote:
> >>>Ideally the interface used by DMA controllers should be identical to
> >>>the interface used by CPUs to talk to the rest of the system: it's
> >>>exactly the same bus interface in hardware, after all.
> >>
> >>I thought we were talking about the interface between the DMA ctrl
> >>and the I/O (devices). Not between the DMA and the "memory" bus system.
> >
> >In hardware (at least for AXI) they're the same thing. A DMA
> >controller is a bus master, just like a CPU. They don't care
> >whether the slave is RAM or a device, they're just issuing
> >memory transactions to addresses.
> It looks like the AXI stream interface also includes a word array.
> I can't tell though whether this is just a decomposed scatter/gather
> list though.


IIRC the word array thing is device specific, not really AXI stream.
I think the whole connection to AXI is a bit unfortunate, these devices
are pretty much the same devices that in other contexts where connected
to other bus standards. Xilinx choose to name them AXI-xxx and I used
the name in our models but I didn't really model anything that is AXI
stream specific..

> There doesn't appear to be a notion of an address though.  You could
> make all operations go to address 0 though but it makes me wonder if
> that's stretching the concept of DMA a bit too much.

Yes, IMO we need a different abstraction..


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