Modify USB device descriptors

Question

I'm currently trying to setup a linux system with a USB OTG Port in device mode. When enabling the NCM gadget with the precomposed configuration, I already could load the gadget driver and a host attached to the OTG port recognized the NCM gadget. Two interfaces with several endpoints where listed under /sys/bus/usb/devices/3-1/. Now, I would like to modify the interfaces (and endpoints) offered to the host.

From my research, I assume, that libcomposite and configfs is the way to go for this. I therefore enabled NCM configurable through configfs in buildroot.

For configuration, I try to follow the kernel documentation about configuring gadgets through the configfs: https://www.kernel.org/doc/Documentation/usb/gadget_configfs.txt

I loaded the libcomposite.ko and mounted the configfs. I could create a folder under /sys/kernel/config/usb_gadget/. When entering the folder, it contains the expected files and subfolders:

total 0
-rw-r--r--    1 root     root        4.0K Feb 16 15:15 UDC
-rw-r--r--    1 root     root        4.0K Feb 16 15:15 bDeviceClass
-rw-r--r--    1 root     root        4.0K Feb 16 15:15 bDeviceProtocol
-rw-r--r--    1 root     root        4.0K Feb 16 15:15 bDeviceSubClass
-rw-r--r--    1 root     root        4.0K Feb 16 15:15 bMaxPacketSize0
-rw-r--r--    1 root     root        4.0K Feb 16 15:15 bcdDevice
-rw-r--r--    1 root     root        4.0K Feb 16 15:15 bcdUSB
drwxr-xr-x    2 root     root           0 Feb 16 14:43 configs
drwxr-xr-x    2 root     root           0 Feb 16 14:43 functions
-rw-r--r--    1 root     root        4.0K Feb 16 15:15 idProduct
-rw-r--r--    1 root     root        4.0K Feb 16 15:15 idVendor
drwxr-xr-x    2 root     root           0 Feb 16 14:43 os_desc
drwxr-xr-x    2 root     root           0 Feb 16 14:43 strings

My problem now is, that I cannot create any function. Creating a folder under ./functions gives me

mkdir functions/ncm.usb3
mkdir: can't create directory 'functions/ncm.usb3': No such file or directory

However, under /lib/modules/4.14.78-linux-v4.14-BSP0112/kernel/drivers/usb/gadget/function/ a file usb_f_ncm.ko does exist!

I have tried several names (e.g. usb0, usb, u1,...), other gadgets (like the mass storage gadget) all without success. I can create configurations under ./configs and also subfolders under ./strings... What piece in the puzzle am I missing here? Any help is appreciated

Side info: A this is an embedded system, modprobe is not available, only insmod...

Update I could solve the problem from above. Previously, I had the libcomposite and ncm as modules (M) in buildroot. When directly including them into the kernel (*) I could add the ncm functionality. This probably is related to the missing modprobe functionality.. However, only four configuration files appear in the created function folder:

total 0
-rw-r--r--    1 root     root        4.0K Feb 16 16:30 dev_addr
-rw-r--r--    1 root     root        4.0K Feb 16 16:30 host_addr
-r--r--r--    1 root     root        4.0K Feb 16 16:30 ifname
-rw-r--r--    1 root     root        4.0K Feb 16 16:30 qmult

How can I specify further attributes of this interface? E.g. Endpoints?

Answer 1

As I did some more research, I would like to answer my own question.

In short: No, apart from what is offered in the configfs, no further configuration of the NCM function is possible. The remaining USB descriptors like endpoints, Packet size, etc. is defined in the source code of the function.

The interfaces the host sees correspond to the functions associated with the gadget on the device side. Multiple functions (like MIDI, mass storage gadget, NCM, etc.) can be associated to the same gadget as described in the kernel configuration docu for the gadget driver. Those will appear as interface on the host side (Some functions like the NCM will show as more than one interface).