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

From: Peter Maydell
Subject: Re: [Qemu-devel] [RFC] QOMification of AXI stream
Date: Mon, 11 Jun 2012 16:34:27 +0100

On 11 June 2012 16:03, Anthony Liguori <address@hidden> wrote:
> On 06/11/2012 09:53 AM, Peter Maydell wrote:
>> 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.
> 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.

Yeah, I've probably been confusing things a little there, sorry.
AXI-Stream is an addressless data stream (not necessarily point-to-point,
the protocol includes TDEST signals to indicate the 'destination' for
a data stream, so you could have a multiple-master-multiple-slave config,
or a config with a slave that accepted several incoming streams over
the same interface). AXI is the general address-based interface.

(Have just read the spec, and AXI-Stream has a number of interesting
features including optional end-of-packet signals, and the ability to
say "these bytes in the data stream are to be ignored" so you can have
a transfer which updates eg bytes 1-5 and 8-15 at the destination
but leaves 6-7 untouched.)

-- PMM

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