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

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2012-06-14 at 04:03 +0200, Edgar E. Iglesias wrote:
> Thanks for the clarificatino Ben.
> 
> I don't know much about PCI but in the embedded world I've never seen
> anything that resemblems what you describe. Devices at the bottom of
> the hierharcy (or at any location) that make acceses to the memory system
> do it through a different port located at a differnt logical position in
> the hierarchy. 

Right, that's what I meant when I said that master and slave interfaces
don't have to be the same, but that's not a problem. A given device
could depend on two memory regions, one for downstream accesses only and
one for upstream accesses, both located in different positions in the
hierarchy.

> In fact, a DMA ctrl can through it's master port access it's
> slave port without it even noticing that it is the same device accessing
> itself and the access will travel top-down through the bus hierarchy.

That sometimes work ... and sometimes doesn't ... some combinations of
busses/devices will not allow that, some will deadlock, it really
depends on the setup.

> I've never seen address decoding beeing done in reverse.. bottom-up.

It really depends on the bus type and happens on embedded as well.

If your device master "port" is directly on the processor bus, you still
have to deal with sibling devices potentially decoding right ? Now, what
about you have a bridge from that bus to another bus. For example ppc
PLB to AXI (that stuff exist out on the field).

An AXI device might issue a cycle on the AXI portion, that can be
decoded by either a sibling AXI device ... or go up. In most cases
though, "upstream" is some kind of substractive decoding (ie. anything
-not- decoded by a sibling goes up).

This gets even more obvious with PCI of course. Then add a b43 wifi card
on PCI, you have AXI -> PCI -> SSB (silicon backplane). Now what happens
when one of the sub-devices on SBB (the MAC interface for example) does
a bus master ? You have at least 2 layers to cross before you hit your
processor bus. 

Add iommu's to the soup and you get into a serious mess :-)

Cheers,
Ben.