uinput: ioctls for UI_BEGIN_FF_UPLOAD fails (returns -1). How to debug?

[Manuel Reimer <mail+linux-input@m-reimer.de>]

Hello,

I'm writing on an usermode input driver.

Currently my force feedback support just consists of some printf's as a 
first try.

My problem is, that it seems like as soon as I first sent something to 
my open uinput device (some buttons that were pressed on the device) the 
uinput backend seems to get into some bad state.

My "force feedback thread" gets the "EV_UINPUT" event trigger, but if I 
try to call

ioctl(args->fduinput, UI_BEGIN_FF_UPLOAD, &upload);

in this case, then the return value is "-1"...

The same code works without any problem if no buttons are pressed and 
"fftest" is called directly after startup of the driver application.

Can someone help me with debugging this problem? My understanding is, 
that this call shouldn't return -1 if the "EV_UINPUT" event was 
successfully received, right?

My system:

$ uname -a
Linux manuelspc 4.4.5-1-ARCH #1 SMP PREEMPT Thu Mar 10 07:38:19 CET 2016 
x86_64 GNU/Linux

Thank you very much in advance.

Manuel