On Sunday 28 January 2007 12:54 pm, Alan Stern wrote:
> On Sun, 28 Jan 2007, jing xiang wrote:
> 
> > Got the idea. But could we re-use the current gadget drivers in such
> > architecture? Is it possible to make the composite layer transparent
> > to the function drivers? 

Those are the two questions; see below for answers ...


> > In theory, the composite layer could get all 
> > the descriptor/configuration/interface information from the function
> > drivers. Thus we could make a little modification to the gadget
> > drivers, and compose different kind of composite devices easily. 

"Easily" in some cases.  For example, you may have a peripheral
controller that doesn't support the composition you want.


> > And 
> > for gadget/function drivers, it could either be one pure function or
> > combine with others to form composite device. We have tried to
> > implement such composite on PXA27x UDC, but met problems as mentioned
> > in last mail.
> 
> In principle this is possible.  It's easy to see why:  If you can combine 
> two function drivers with each other to make a composite gadget, then you 
> combine one of them with nothing, resulting in a non-composite gadget with 
> only one function.
> 
> However none of the existing drivers were written with this sort of thing
> in mind.  They would have to be modified before they could fit into the
> composite-gadget scenario.  So the answer to your first question is "No",

"No, not as-is ... they need modification first.  Most of the existing
code could be easily modified though."  The non-control endpoints should
work almost identically, and that's the bulk of the functionality.


> and the answer to the second question is "Yes, but only if the function
> drivers are written properly".

Erm, "No, because the function drivers _know_ they are not taking over
ownership of the whole gadget, since they are making an effort to share
it ... and such sharing is never needed without such a layer."

- Dave


> 
> Alan Stern
> 

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