Class-specific interfaces in FunctionFS gadget

[Marian <n@xxxxxx>]

Hello,

I am currently trying to implement a more complex USB UAC2 audio device 
than the one provided by `f_uac2` using the FunctionFS gadget `f_fs`. 
However, as far as I can tell, this currently won't work because 
FunctionFS parses the supplied descriptors in `ffs_do_single_desc`, 
which does not handle any class-specific interfaces. Adding support 
there is also non-trivial because of the relatively large number of 
descriptor (sub)types that would need to be parsed and handled just for 
UAC1 and 2 alone. As I understand it (correct me if I'm wrong), 
FunctionFS needs to parse them to make the descriptors "relocatable", 
i.e. support offsetting interface, endpoint and string references once 
the USB gadget is actually initialized in case there are other functions 
in the configuration, making this parsing essential for the gadget to work.

I was wondering what the best option to support more/any descriptors in 
FunctionFS would be or if I'm missing something. Would it not be better 
to have the user of `f_fs` (optionally) supply a sort of "relocation 
table" containing (byte_offset, entity_type) entries alongside the 
already supplied descriptor and string blobs? This could be used as an 
alternative to

Unrelated to that, it does currently support interface association 
descriptors, but ignores `bFirstInterface`. Is this correct? If an 
`f_fs` instance would e.g. start at interface number 3, 
`bFirstInterface` should be offset by 3, no?

Best regards,
Marian