[General Questions and Answers] thorstenhirsch:
Several guides explain how to install APX drivers "on Windows only", and that they are not required on Linux (e.g. this one). Unfortunately I think that I do require such a driver on Linux, because I want to pass the APX device to my VirtualBox guest (where Win7 is running). And I think I can only pass usb devices to the guest that appear in VirtualBox's usb device list - which is a list of devices that have a Linux driver. So here's the question:

How can I pass an APX device to a VirtualBox guest (Win7) on a Linux host?

My APX device is a LG P990. It's based on nvidia tegra, so dmesg says:

[ 7386.292082] usb 2-1: new high-speed USB device number 13 using ehci_hcd
[ 7386.424943] usb 2-1: New USB device found, idVendor=0955, idProduct=7f20
[ 7386.424954] usb 2-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 7386.424961] usb 2-1: Product: APX
[ 7386.424967] usb 2-1: Manufacturer: NVIDIA Corp.

And this APX device actually is already being passed to VirtualBox, because an error window pops up (not a Windows window, but a VirtualBox window):

Failed to attach the USB device NVIDIA Corp. APX [0103] to the virtual machine Win7.
Failed to create a proxy device for the USB device. (Error: VERR_READ_ERROR).

Result Code:
NS_ERROR_FAILURE (0x80004005)
Component:
Console
Interface:
IConsole {db7ab4ca-2a3f-4183-9243-c1208da92392}

Might be a permission problem. However, I don't know how to solve it, because there's no device in /dev that appears when plugging in the LG P990. Google says that my user needs to be in the vboxusers group - but he already is. All the virtualbox devices in /dev are also accessible with my user. So in the end I think that my user should be able to access all devices that are available as a "file" in /dev. But I think there's something missing in /dev. It might be either...

a) /dev/ttyUSB0, which is the device of the usbserial module. It is not being loaded automatically, but I can force it with "sudo modprobe usbserial vendor=0x0955 product=0x7f20". But this doesn't seem to be the correct driver, because it says that the device has been disconnected as soon as the module is loaded. These dmesg entries appear all at once:

[ 4187.867971] usbcore: registered new interface driver usbserial
[ 4187.867997] usbcore: registered new interface driver usbserial_generic
[ 4187.869407] USB Serial support registered for generic
[ 4187.869436] usbserial_generic 2-1:1.0: The "generic" usb-serial driver is only for testing and one-off prototypes.
[ 4187.869440] usbserial_generic 2-1:1.0: Tell linux-usb@vger.kernel.org to add your device to a proper driver.
[ 4187.869443] usbserial_generic 2-1:1.0: generic converter detected
[ 4187.870237] usb 2-1: generic converter now attached to ttyUSB0
[ 4187.870251] usbserial: USB Serial Driver core
[ 4188.312867] usb 2-1: USB disconnect, device number 7
[ 4188.313295] generic ttyUSB0: generic converter now disconnected from ttyUSB0
[ 4188.313319] usbserial_generic 2-1:1.0: device disconnected

So with the usbserial drivers there's neither /dev/ttyUSB0 nor a USB device in VirtualBox (the APX device that was available without any driver has disappeared).

b) /dev/ttyACM0 is the device of the module cdc_acm, which I think might could also handle the APX device; this is what happens in dmesg when attaching the LG to the USB port while cdc_acm is loaded (and usbserial is not loaded):

[ 4323.356889] usbcore: registered new interface driver cdc_acm
[ 4323.356895] cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters

But nothing is happening at all. Again, there's no /dev/ttyACM0. And this time the APX device in VirtualBox is still there (with the VERR_READ_ERROR again). So cdc_acm seems to have no effect at all.

c) loading usbserial AND cdc_acm - the same as if usbserial is loaded without cdc_acm; cdc_acm has no effect, so the same happens as in a)

Now I'm stuck. I don't know how to fix the permission error without a proper device in /dev. My best guess was the usbserial device that should have created /dev/ttyUSB0, but doesn't, and even prevents the APX "raw device" (without driver) to appear in VirtualBox's usb device list.