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

[Anthony Liguori <anthony@codemonkey.ws>]

On 06/08/2012 09:59 PM, Paul Brook wrote:
>>>>> Of course we then hit the usual problem with QOM that we can only link
>>>>> to objects, and it's impossible to expose multiple interfaces of the
>>>>> same type.
>>>>
>>>> I'm pretty sure Anthony claimed this was entirely possible --
>>>> presumably that's how Pins are going to work.
>>>
>>> Really?  Every time I've talked to him I've got the opposite impression.
>>> Part of the response has been that interrupt pins are the only case
>>> where this actually occurs, so It's not worth fixing properly.
>>
>> I think it depends on your definition of "properly".
>>
>> There's really only three concepts in QOM that matter for this discussion:
>> 1) objects 2) children and 3) links.
>>
>> There is absolutely no difference between a Pin object and a SerialState
>> object. They both are first-class objects are far as QOM and concerned.
>> Both can have links to other objects.
>>
>> The most common way for other objects to create objects is via children.  A
>> device could have a bunch of Pin child objects with that being the sole
>> communication mechanism with the outside world.
>
> And those pin objects would presumably communicate back to the device via some
> as-yet unimplemented PinMultiplex interface link?  Or are you expecting the
> Pin objects to have an API call that allows the device to register an
> arbitrary QEMUBH (or equivalent)?

Notifiers actually.  The rough model is Signal/Slots similar to GObject and/or QT.

>
>> A device could also have a 'PCISocket' child object (which inherits from
>> PCIDevice) in order to expose a PCI interface to the world.
>>
>> For most bus-based devices, I think the above is poor design.  But that's
>> my opinion from a modeling PoV, QOM doesn't have an opinion from an
>> infrastructure PoV.
>
> So what is a good design?  Are you hoping most of your interfaces are
> stateless, so can be implemented directly on the device object?

QOM is not a panacea.  We still need to have good objects/interfaces.  For this 
specific example, it sounds like a job for the DMA API dwg/benh have proposed. 
I responded with more detail in another part of the thread.

Regards,

Anthony Liguori

>
> Paul
>