How to get the kernel to emulate force feedback events?

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

Hello,

if I plug my Xbox 360 controller and run fftest on it, then the 
following force feedback modes are listed as supported:

| Force feedback effects types: Periodic, Rumble, Gain,
| Force feedback periodic effects: Square, Triangle, Sine,

If I have a look at the driver itself, then it actually only supports 
FF_RUMBLE:

https://github.com/torvalds/linux/blob/master/drivers/input/joystick/xpad.c#L1054
https://github.com/torvalds/linux/blob/master/drivers/input/joystick/xpad.c#L966

So somewhere in the kernel source there has to be some kind of built-in 
force feedback effect emulation...

If I do the same in an uinput driver (only report "FF_RUMBLE" as being 
supported), then fftest only shows this one effect to be supported by 
the device. The "kernel side emulation" does not kick in...

The kernel documentation 
(https://www.kernel.org/doc/Documentation/input/ff.txt) says:

| Note: In most cases you should use FF_PERIODIC instead of FF_RUMBLE.
|       All devices that support FF_RUMBLE support FF_PERIODIC (square,
|       triangle, sine) and the other way around.

But what could be the mistake, which allowed me to create a device 
driver which actually only supports FF_RUMBLE? Are there any special 
driver parameters that have to be present, so the kernel side emulation 
gets active?

Thanks in advance.

Best regards,

Manuel