[Xcsoar-devel] USB to Serial Adapter Support

[Felix H. <fel...@we...>]

Hello,

you know that I had developed an integration of the
usb-serial-for-android library (respectively a forked version of the
library, see
https://code.google.com/r/felixhaedicke-usb-serial-for-android) last
year. The code can still be found in my git repository, branch
usb-serial-for-android. But it is not based on the current XCSoar code
any more and would need some bugfixing.

But I do not like the Android USB API. The main reason is that support
for asynchronous reading / writing is unusable. See
https://code.google.com/p/android/issues/detail?id=39522. For reading
data, polling is required all the time, using
UsbDeviceConnection.bulkTransfer().

>From my point of view, libusb offers by far the better API for userspace
USB drivers. I have already implemented a USB Serial Adapter driver
library based on libusbserial. It is a pure C library, and the code can
be found here: https://github.com/felixhaedicke/libusbserial.

libusb also works on Android, although it is not an official Android
API. For enumerating devices, I would still use the Android USB API, for
one simple reason: The permission for accessing a USB device needs to be
requested using the Android APIs. The native file descriptor can be
accessed using the UsbDeviceCOnnection.getFileDescriptor() method. To
hand over a UsbDeviceConnection (Android API) to libusb, libusb needs to
be patched (to open a libusb_device_handle using the file descriptor).

The libusb based code could also be used on other platforms. This is not
very helpful on Linux and Windows, because there are kernel drivers on
these platforms. But the code could also be used on iOS. At least some
iPad devices appear to be USB-OTG compatible and some people are
successfully using libusb on it. Unfortunately, I cannot verify this,
because I don't have an iPad.

What do you think? Should I rather complete / renew the Java code or
should I integrate my libusbserial library?

Regards
Felix