[PATCH libevdev 2/4] Put a warning in the documentation about disabling/enabling event types/codes

[David Herrmann]

Hi

On Thu, Apr 21, 2016 at 11:59 PM, Peter Hutterer
<peter.hutterer at who-t.net> wrote:
> If a client disables a code halfway through an MT interaction the touch up
> events are lost. If it re-enables a code it may get weird event sequences.
> That's something the client has to deal with though because without getting
> events a client cannot know the current state of a device.

I always thought we cache the state and discard anything that does not
involve a state-change? So if a key-down is "not known" to us, a
following "key up" event is silently discarded? According to your
comments here, this is not the case, right? Or is this only related to
ABS slots?
I should refresh my memory on evdev more often...

David

> Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
> ---
>  libevdev/libevdev.h | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/libevdev/libevdev.h b/libevdev/libevdev.h
> index 3edc3ae..856aeb9 100644
> --- a/libevdev/libevdev.h
> +++ b/libevdev/libevdev.h
> @@ -1751,6 +1751,12 @@ void libevdev_set_abs_info(struct libevdev *dev, unsigned int code, const struct
>   * This is a local modification only affecting only this representation of
>   * this device.
>   *
> + * @note If a caller has previously invoked libevdev_disable_event_code() or
> + * libevdev_disable_event_type(), re-enabling an event type may cause
> + * unexpected event sequences. For example, re-enabling an event type while a
> + * touch is down may cause a touch end to be passed to the caller without a
> + * preceding touch down event.
> + *
>   * @param dev The evdev device, already initialized with libevdev_set_fd()
>   * @param type The event type to enable (EV_ABS, EV_KEY, ...)
>   *
> @@ -1778,6 +1784,11 @@ int libevdev_enable_event_type(struct libevdev *dev, unsigned int type);
>   * This is a local modification only affecting only this representation of
>   * this device.
>   *
> + * @note This function should only be invoked when the respective event type
> + * is in a neutral state. Otherwise, a caller may get unexpected event
> + * sequences. For example, disabling an event type while a touch is down may
> + * cause the touch end event to be discarded.
> + *
>   * @param dev The evdev device, already initialized with libevdev_set_fd()
>   * @param type The event type to disable (EV_ABS, EV_KEY, ...)
>   *
> @@ -1811,6 +1822,12 @@ int libevdev_disable_event_type(struct libevdev *dev, unsigned int type);
>   * that already has the given event code enabled, the values in data
>   * overwrite the previous values.
>   *
> + * @note If a caller has previously invoked libevdev_disable_event_code() or
> + * libevdev_disable_event_type(), re-enabling an event code may cause
> + * unexpected event sequences. For example, re-enabling an event code while a
> + * touch is down may cause a touch end to be passed to the caller without a
> + * preceding touch down event.
> + *
>   * @param dev The evdev device, already initialized with libevdev_set_fd()
>   * @param type The event type to enable (EV_ABS, EV_KEY, ...)
>   * @param code The event code to enable (ABS_X, REL_X, etc.)
> @@ -1841,6 +1858,11 @@ int libevdev_enable_event_code(struct libevdev *dev, unsigned int type, unsigned
>   * Disabling codes of type EV_SYN will not work. Don't shoot yourself in the
>   * foot. It hurts.
>   *
> + * @note This function should only be invoked when the respective event type
> + * is in a neutral state. Otherwise, a caller may get unexpected event
> + * sequences. For example, disabling an event code while a touch is down may
> + * cause the touch end event to be discarded.
> + *
>   * @param dev The evdev device, already initialized with libevdev_set_fd()
>   * @param type The event type to disable (EV_ABS, EV_KEY, ...)
>   * @param code The event code to disable (ABS_X, REL_X, etc.)
> --
> 2.7.3
>
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