Along with all the issues involved in autoconfiguring endpoint selection, the functions in a composite gadget will have to share other resources: interface numbers and string descriptors.
Right. But you can autoconfigure endpoints (at least for bulk-only drivers) and not support composite configurations. The converse is also true, although it'd be nicer to build on endpoint autoconfiguration for composite configuration support.
For interface numbers, the function driver will have to tell the main routine how many interfaces it wants for each configuration. In return the main routine will tell it what interface numbers it can use, and the function driver must update its descriptors accordingly. It's possible to create a simple library routine to update the interface numbers in all the standard descriptors, but if a function uses class- or vendor-specific descriptors with embedded interface numbers then it will have to update them by itself.
Yes.
Similar considerations apply to string descriptor numbers. But with strings there's another issue to contend with: language selection. As I understand it, in a USB device each string is supposed to be available in all the languages supported by that device. This means that every function in a composite gadget would have to support a common set of languages, and the main routine would have to know what this set is.
That's my understanding too.
Unlike the other problems, this one can't be solved without explicit coordination between the function drivers. So far as I know, there aren't yet any gadget drivers that support more than one language. But when they do appear this will be a point of contention.
Particularly for languages that can't use ISO-8859-1 for their strings; they'll need to invent some other way to package strings, since C-strings won't behave!
I'd be happy with an initial framework that worked well with a single language, but required minor out-patient surgery to handle multiple languages. Folk doing multi-language products will be working with custom systems anyway, they'll be paying such costs in other places too.
- Dave
Alan Stern
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