[PATCH libinput] tablet: add a fuzz-filter to avoid spamming callers with subpixel updates

[Jason Gerecke]

On Mon, Apr 11, 2016 at 5:17 PM, Peter Hutterer
<peter.hutterer at who-t.net> wrote:
> This is especially a problem for the cursor tool which can be legitimately
> left on the tablet. It wobbles by a couple of device units, resulting in
> continuous axis updates to the caller. Pre-filter any of these events by the
> axis' fuzz value so we don't even process them.
>
> For ABS_DISTANCE which doesn't have a fuzz we hard-code a minimum fuzz of 2.
> This should eventually land in the udev hwdb though.
>

Bikeshed: s/udev hwdb/kernel/ (and poke Ping and I with a stick) since
the lack of fuzz isn't a huge issue but is still something that should
be fixed?

Reviewed-by: Jason Gerecke <jason.gerecke at wacom.com>

Jason
---
Now instead of four in the eights place /
you’ve got three, ‘Cause you added one  /
(That is to say, eight) to the two,     /
But you can’t take seven from three,    /
So you look at the sixty-fours....

> https://bugs.freedesktop.org/show_bug.cgi?id=94892
>
> Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
> ---
>  src/evdev-tablet.c | 35 +++++++++++++++++++++++++++++++++++
>  src/evdev-tablet.h |  2 ++
>  2 files changed, 37 insertions(+)
>
> diff --git a/src/evdev-tablet.c b/src/evdev-tablet.c
> index 84563a8..be828d9 100644
> --- a/src/evdev-tablet.c
> +++ b/src/evdev-tablet.c
> @@ -112,6 +112,36 @@ tablet_device_has_axis(struct tablet_dispatch *tablet,
>         return has_axis;
>  }
>
> +static inline bool
> +tablet_filter_axis_fuzz(const struct tablet_dispatch *tablet,
> +                       const struct evdev_device *device,
> +                       const struct input_event *e,
> +                       enum libinput_tablet_tool_axis axis)
> +{
> +       int delta, fuzz;
> +       int current, previous;
> +
> +       previous = tablet->prev_value[axis];
> +       current = e->value;
> +       delta = previous - current;
> +
> +       fuzz = libevdev_get_abs_fuzz(device->evdev, e->code);
> +
> +       /* ABS_DISTANCE doesn't have have fuzz set and causes continuous
> +        * updates for the cursor/lens tools. Add a minimum fuzz of 2, same
> +        * as the xf86-input-wacom driver
> +        */
> +       switch (e->code) {
> +       case ABS_DISTANCE:
> +               fuzz = max(2, fuzz);
> +               break;
> +       default:
> +               break;
> +       }
> +
> +       return abs(delta) <= fuzz;
> +}
> +
>  static void
>  tablet_process_absolute(struct tablet_dispatch *tablet,
>                         struct evdev_device *device,
> @@ -137,6 +167,11 @@ tablet_process_absolute(struct tablet_dispatch *tablet,
>                         break;
>                 }
>
> +               tablet->prev_value[axis] = tablet->current_value[axis];
> +               if (tablet_filter_axis_fuzz(tablet, device, e, axis))
> +                       break;
> +
> +               tablet->current_value[axis] = e->value;
>                 set_bit(tablet->changed_axes, axis);
>                 tablet_set_status(tablet, TABLET_AXES_UPDATED);
>                 break;
> diff --git a/src/evdev-tablet.h b/src/evdev-tablet.h
> index 880d523..d12c675 100644
> --- a/src/evdev-tablet.h
> +++ b/src/evdev-tablet.h
> @@ -55,6 +55,8 @@ struct tablet_dispatch {
>         unsigned char changed_axes[NCHARS(LIBINPUT_TABLET_TOOL_AXIS_MAX + 1)];
>         struct tablet_axes axes;
>         unsigned char axis_caps[NCHARS(LIBINPUT_TABLET_TOOL_AXIS_MAX + 1)];
> +       int current_value[LIBINPUT_TABLET_TOOL_AXIS_MAX + 1];
> +       int prev_value[LIBINPUT_TABLET_TOOL_AXIS_MAX + 1];
>
>         /* Only used for tablets that don't report serial numbers */
>         struct list tool_list;
> --
> 2.5.5
>