File transfer is not yet enabled in the released kernels. The reasons for this are:
- The storage device gadget driver (which would be the "standard way") is not tested yet.
- The storage device gadget driver accesses the storage as raw block devices, and thus requires exclusive access to them. In other words: the filesystem is mounted by the host PC. This is why you can see the ext2/ext3 partition from a linux PC despite the fact that the original firmware does not support it.
- Having a serial console is a MUST for application developers. This is provided by the serial gadget driver, which is enabled now (and has a hideous bug that crashes the kernel when unplugging the USB cable).
- You can't use more than one gadget device simultaneously in the 18.104.22.168 kernel. Though I think this is solved in recent kernels by implementing composite gadget devices, backporting it to the 22.214.171.124 kernel is a huge task, and even if that is achieved, the composite device we would need (serial + storage) is not implemented yet even in the latest kernel.
I can think of several ther alternative ways to get what we need:
- Using a serial file transfer protocol over the serial gadget. This is rather inconvenient and definitely not for the end user. I know next to nothing about OBEX, but I'm given to understand that it can work also over a serial line. Maybe its usage would be simpler that a classic protocols for the end user.
- Module loading/unloading of the serial and storage gadget devices. This requires enabling module loading (which enlarges the kernel), is cumbersome to implement, and probably requires fixing first the above mentioned unplug bug.
- Establishing a network connection, either by using the ethernet gadget device or by using SLIP over the serial gadget (the first would be easier). Once the network connection is established you can telnet to get console access and you can transfer files via FTP or SCP. This requires enabling the kernel and user space networking stuff, and would probably be a bit inconvenient for the end user, since it would require to configure the gadget network card.
All your comments and suggestions are welcome.