linux-kernel - Re: uinput: waiting for UI_FF_UPLOAD events will not inform user when allocation is required

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Message-ID: <ZSm9zeepuZ94A-ZI@google.com>
Date:   Fri, 13 Oct 2023 14:59:41 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     John Salamon <salamonj9@...il.com>
Cc:     rydberg@...math.org, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: uinput: waiting for UI_FF_UPLOAD events will not inform user
 when allocation is required

Hi John,

On Tue, Oct 10, 2023 at 05:38:27PM +1030, John Salamon wrote:
> Currently the "fake" input events generated by uinput in response to
> effect uploads will return an effect with an id that has already been
> handled by input_ff_upload in ff-core.c, which can modify the effect
> id. This causes a problem specifically when the effect originally
> uploaded via the EVIOCSFF ioctl contained an effect with -1, as the
> userspace code handling UI_FF_UPLOAD receives an effect with an id
> other than -1, and therefore will not know an allocation was
> requested.

The kernel never changes ID of an existing effect, the only time ID is
changed is when userspace indicates that a new effect should be created
by setting effect ID to -1.

The handler of force feedback effects should know what effects (with
what IDs) have been uploaded to the device so far, so whenever it sees a
request for an effect with previously unseen effect_id it should
recognize this as a signal that a new effect/id has been allocated by
the kernel.

> 
> I notice that the "old" field on the ff_effect struct is set to NULL
> when the -1 id is changed (in input_ff_upload), which can serve as a
> flag that an allocation was requested. If it is the intention is that
> uinput users check if old == NULL to know when allocations are needed
> I think uinput documentation should describe this.

No, not really, as explained above.

> 
> I first noticed this using python-evdev, see my issue report here:
> https://github.com/gvalkov/python-evdev/issues/199

Thanks.

-- 
Dmitry