I was able to get some more information, but as yet have not been able to
 get time on the real hardware. Attached is a 2nd patch which should
improve the support, I'll formally submit it next week.... But if anyone
can test it first that would be great.

Obviously the 'submit next week' never happened... but I now have one of
these wheels to work on. The bad news is that the information we got does
not appear to be complete.

At present the wheel is mis-recognised as a G27 and forced into an
emulation state, it is somewhat usable in that mode.

If we send the 'G29 mode' command the wheel comes back with a device with
2x HID interfaces and that appears to screw up the probe (full log
attached).
--
Sep 28 12:30:33 retrobox kernel: [11079.944056] usb 2-1: new full-speed
USB device number 3 using uhci_hcd
Sep 28 12:30:33 retrobox kernel: [11080.113102] usb 2-1: New USB device
found, idVendor=046d, idProduct=c24f
Sep 28 12:30:33 retrobox kernel: [11080.113113] usb 2-1: New USB device
strings: Mfr=1, Product=2, SerialNumber=0
Sep 28 12:30:33 retrobox kernel: [11080.113120] usb 2-1: Product: G29
Driving Force Racing Wheel
Sep 28 12:30:33 retrobox kernel: [11080.113125] usb 2-1: Manufacturer:
Logitech
Sep 28 12:30:33 retrobox kernel: [11080.124431] input: Logitech G29
Driving Force Racing Wheel as
/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1:1.0/0003:046D:C24F.0002/input/input10
Sep 28 12:30:33 retrobox kernel: [11080.125134] logitech
0003:046D:C24F.0002: input,hidraw0: USB HID v1.11 Joystick [Logitech G29
Driving Force Racing Wheel] on usb-0000:00:1d.0-1/input0
Sep 28 12:30:33 retrobox kernel: [11080.126094] logitech
0003:046D:C24F.0002: Force feedback support for Logitech Gaming Wheels
Sep 28 12:30:33 retrobox kernel: [11080.134307] logitech
0003:046D:C24F.0003: hiddev0,hidraw1: USB HID v1.11 Device [Logitech G29
Driving Force Racing Wheel] on usb-0000:00:1d.0-1/input1
Sep 28 12:30:33 retrobox kernel: [11080.134319] logitech
0003:046D:C24F.0003: not enough values in HID_OUTPUT_REPORT 0 field 0
Sep 28 12:30:33 retrobox kernel: [11080.134339] logitech: probe of
0003:046D:C24F.0003 failed with error -1
--

Any suggestions as to how we can 'ignore' or unbind the 2nd interface in
order to only work with the 1st?
Simon