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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 16:58:17 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Mon, Jun 11, 2012 at 03:53:23PM +0100, 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.

In many cases they are not the same thing. The DMA controller has
an interface to the bus (the one you refer to) and in many cases
dedicated channels connected to the various devices. These channels
may or may not resemble the bus a CPU uses, typically they don't.

There are of course systems of any kind, even ones that use the
system bus to transfer data from the DMA to/from the devices.

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