Forwarding EV_UINPUT events for FF is difficult due to undocumented behaviour

[human9]

I am unsure if this is really an issue of python-evdev but I've discovered it using python-evdev, so I'll document it here.

I am in the situation of creating a ff-capable uinput device, and forwarding ff events from this device back to an original one. So, this and this in the same code.

Forwarding events directly will fail. This is because an effect id of -1 is never sent to the uinput device under UI_FF_UPLOAD, and as per kernel ff documention an id of -1 is required to allocate the new effect.

Why is a value of -1 never sent through? Because the -1 is actually modified in place by the kernel's handling in ff-core.c.

So, what to do? My workaround is to read the old field that is set to NULL here. That seems a reliable indicator of when an allocation has been requested. In a python program, the relevant snippet might look like:

# Handling EV_UINPUT events from our uinput_dev here
if event.code == e.UI_FF_UPLOAD:
	
    upload = uinput_dev.begin_upload(event.value)
    
    # Testing if upload.old was set to NULL
    # It's not a null pointer here but at some point the null makes this all zeros
    if all(x == 0 for x in bytearray(upload.old)):
        upload.effect.id = -1    

    # Before uploading to a real device
    real_dev.upload_effect(effect)
    
    upload.retval = 0
    uinput_dev.end_upload(upload)

Handling it this way results in fully functional forwarding of all FF events.

Happy to submit a pull request for documentation if desired. Although, I'm not sure if this is intended behaviour or a bug in the way uinput handles this.. It really seems like it should forward the -1 id values, so I might report upstream.

[human9]

I asked upstream and this is apparently all working as intended. Below you can find a test script that should give a functional rumble. It checks for whether an unseen effect id comes through to know when to ask for a new effect to be allocated (done by setting id to -1). See upstream discussion.

import evdev
from evdev import ecodes as e

input_device = evdev.InputDevice("/dev/input/event12") # replace with whatever your FF-capable device is
uinput = evdev.UInput.from_device(input_device, filtered_types=[e.EV_SYN])

# Keeps track of which effects have been uploaded
effects = set()

for event in uinput.read_loop():
    
    # Handle the special uinput events
    if event.type == e.EV_UINPUT:

        if event.code == e.UI_FF_UPLOAD:
            upload = uinput.begin_upload(event.value)

            if upload.effect.id not in effects:
                effects.add(upload.effect.id)
                upload.effect.id = -1

            input_device.upload_effect(upload.effect)
            upload.retval = 0
            uinput.end_upload(upload)
            
        elif event.code == e.UI_FF_ERASE:
            erase = uinput.begin_erase(event.value)
            erase.retval = 0
            input_device.erase_effect(erase.effect_id)
            effects.remove(erase.effect_id)
            uinput.end_erase(erase)
    
    # Forward writes to actual rumble device.
    elif event.type == e.EV_FF:
        input_device.write(event.type, event.code, event.value)

I haven't been able to find an example on the internet of anyone actually doing this correctly, so it would be great if it could be documented somewhere.

[sezanzeb]

it would be great if it could be documented somewhere.

Your code works for me. How about appending it to the tutorials section? https://github.com/gvalkov/python-evdev/blob/main/docs/tutorial.rst

[human9]

Happy to do so. Will draft a pull request in a bit.

[human9]

Closing as docs were added in #198