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High-resolution scroll wheel support


Follow up on Peter's work to add high-resolution scroll wheel support !2 (closed)

Starting with Linux Kernel v5.0 two new axes are available for high-resolution wheel scrolling: REL_WHEEL_HI_RES and REL_HWHEEL_HI_RES. Both axes send data in fractions of 120 where each multiple of 120 amounts to one logical scroll event. Fractions of 120 indicate a wheel movement less than one detent. A "detent" is the named used in the Kernel for a mouse wheel click:

Three new events are now available on libinput: LIBINPUT_EVENT_POINTER_SCROLL_WHEEL, LIBINPUT_EVENT_POINTER_SCROLL_FINGER, and LIBINPUT_EVENT_POINTER_SCROLL_CONTINUOUS. These events replace the LIBINPUT_EVENT_POINTER_AXIS event, so new clients should simply ignore that event.

This LIBINPUT_EVENT_POINTER_SCROLL_WHEEL event adds a new API libinput_pointer_scroll_get_value_v120(). The libinput_pointer_scroll_get_value_v120() is a mirror from the kernel API (itself a copy of the Windows API). The new event is sent for all wheel events, even those that don't technically support high-resolution scrolling and even on older kernels that don't have this feature. So callers can simply ignore any LIBINPUT_EVENT_POINTER_AXIS event and use the new event types only.

For LIBINPUT_EVENT_POINTER_SCROLL_FINGER and LIBINPUT_EVENT_POINTER_SCROLL_CONTINUOUS, as well as LIBINPUT_EVENT_POINTER_SCROLL_WHEEL, a new API is available: libinput_pointer_scroll_get_value(). This API is similar to libinput_pointer_axis_get_value().

Status (X11)

For code reviewers

Updating Peter's code, I had to resolve a conflict in calculate_axis_value. scroll_pixel_distance should be working on this branch, but if you can, pay attention to this bit:

static inline bool
calculate_axis_value(struct xf86libinput *driver_data,
		     enum libinput_pointer_axis axis,
		     struct libinput_event_pointer *event,
		     double *value_out)
	} else {
		double dist = driver_data->options.scroll_pixel_distance;
		assert(dist != 0.0);

		value = libinput_event_pointer_get_axis_value(event, axis);
		/* We need to scale this value into our scroll increment range
		 * because that one is constant for the lifetime of the
		 * device. The user may change the ScrollPixelDistance
		 * though, so where we have a dist of 10 but an increment of
		 * 15, we need to scale from 0..10 into 0..15.
		 * We now switched to vdist of 120, so make this
		 * proportionate - 120/15 is 8.
		value = value/dist * SCROLL_INCREMENT * 8;

	*value_out = value;

	return true;

Other than that, this MR is mostly API updates and a little bug fix.

cc @whot

Edited by Peter Hutterer

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