xref: /src/sys/arch/macppc/dev/pbms.c

/* $Id: pbms.c,v 1.20 2022/03/28 12:43:12 riastradh Exp $ */

/*
 * Copyright (c) 2005, Johan Walln
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *   1. Redistributions of source code must retain the above copyright
 *      notice, this list of conditions and the following disclaimer.
 *
 *   2. Redistributions in binary form must reproduce the above
 *      copyright notice, this list of conditions and the following
 *      disclaimer in the documentation and/or other materials provided
 *      with the distribution.
 *
 *   3. The name of the copyright holder may not be used to endorse or
 *      promote products derived from this software without specific
 *      prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER "AS IS" AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
 * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * The pbms driver provides support for the trackpad on new (post
 * February 2005) Apple PowerBooks (and iBooks?) that are not standard
 * USB HID mice.
 */

/*
 * The protocol (that is, the interpretation of the data generated by
 * the trackpad) is taken from the Linux appletouch driver version
 * 0.08 by Johannes Berg, Stelian Pop and Frank Arnold.  The method
 * used to detect fingers on the trackpad is also taken from that
 * driver.
 */

/*
 * To add support for other devices using the same protocol, add an
 * entry to the pbms_devices table below.  See the comments for
 * pbms_devices and struct pbms_devs.
 */

/*
 * PROTOCOL:
 *
 * The driver transfers continuously 81 byte events.  The last byte is
 * 1 if the button is pressed, and is 0 otherwise. Of the remaining
 * bytes, 26 + 16 = 42 are sensors detecting pressure in the X or
 * horizontal, and Y or vertical directions, respectively.  On 12 and
 * 15 inch PowerBooks, only the 16 first sensors in the X-direction
 * are used. In the X-direction, the sensors correspond to byte
 * positions
 *
 *   2, 7, 12, 17, 22, 27, 32, 37, 4, 9, 14, 19, 24, 29, 34, 39, 42,
 *   47, 52, 57, 62, 67, 72, 77, 44 and 49;
 *
 * In the Y direction, the sensors correspond to byte positions
 *
 *   1, 6, 11, 16, 21, 26, 31, 36, 3, 8, 13, 18, 23, 28, 33 and 38.
 *
 * On 12 inch iBooks only the 9 first sensors in Y-direction are used.
 * The change in the sensor values over time is more interesting than
 * their absolute values: if the pressure increases, we know that the
 * finger has just moved there.
 *
 * We keep track of the previous sample (of sensor values in the X and
 * Y directions) and the accumulated change for each sensor.  When we
 * receive a new sample, we add the difference of the new sensor value
 * and the old value to the accumulated change.  If the accumulator
 * becomes negative, we set it to zero.  The effect is that the
 * accumulator is large for sensors whose pressure has recently
 * increased.  If there is little change in pressure (or if the
 * pressure decreases), the accumulator drifts back to zero.
 *
 * Since there is some fluctuations, we ignore accumulator values
 * below a threshold.  The raw finger position is computed as a
 * weighted average of the other sensors (the weights are the
 * accumulated changes).
 *
 * For smoothing, we keep track of the previous raw finger position,
 * and the virtual position reported to wsmouse.  The new raw position
 * is computed as a weighted average of the old raw position and the
 * computed raw position.  Since this still generates some noise, we
 * compute a new virtual position as a weighted average of the previous
 * virtual position and the new raw position.  The weights are
 * controlled by the raw change and a noise parameter.  The position
 * is reported as a relative position.
 */

/*
 * TODO:
 *
 * Add support for other drivers of the same type.
 *
 * Add support for tapping and two-finger scrolling?  The
 * implementation already detects two fingers, so this should be
 * relatively easy.
 *
 * Implement some of the mouse ioctls?
 *
 * Take care of the XXXs.
 *
 */

#include <sys/cdefs.h>

#include <sys/param.h>
#include <sys/device.h>
#include <sys/errno.h>

#include <sys/ioctl.h>
#include <sys/systm.h>
#include <sys/tty.h>

#include <dev/usb/usb.h>
#include <dev/usb/usbdi.h>
#include <dev/usb/usbdevs.h>
#include <dev/usb/uhidev.h>
#include <dev/hid/hid.h>

#include <dev/wscons/wsconsio.h>
#include <dev/wscons/wsmousevar.h>

/*
 * Magic numbers.
 */


/* The amount of data transferred by the USB device. */
#define PBMS_DATA_LEN 81

/* The maximum number of sensors. */
#define PBMS_X_SENSORS 26
#define PBMS_Y_SENSORS 16
#define PBMS_SENSORS (PBMS_X_SENSORS + PBMS_Y_SENSORS)

/*
 * Parameters for supported devices.  For generality, these parameters
 * can be different for each device.  The meanings of the parameters
 * are as follows.
 *
 * desc:      A printable description used for dmesg output.
 *
 * noise:     Amount of noise in the computed position. This controls
 *            how large a change must be to get reported, and how
 *            large enough changes are smoothed.  A good value can
 *            probably only be found experimentally, but something around
 *            16 seems suitable.
 *
 * product:   The product ID of the trackpad.
 *
 *
 * threshold: Accumulated changes less than this are ignored.  A good
 *            value could be determined experimentally, but 5 is a
 *            reasonable guess.
 *
 * vendor:    The vendor ID.  Currently USB_VENDOR_APPLE for all devices.
 *
 * x_factor:  Factor used in computations with X-coordinates.  If the
 *            x-resolution of the display is x, this should be
 *            (x + 1) / (x_sensors - 1).  Other values work fine, but
 *            then the aspect ratio is not necessarily kept.
 *
 * x_sensors: The number of sensors in the X-direction.
 *
 * y_factor:  As x_factors, but for Y-coordinates.
 *
 * y_sensors: The number of sensors in the Y-direction.
 */

struct pbms_dev {
	const char *descr; /* Description of the driver (for dmesg). */
	int noise;	   /* Amount of noise in the computed position. */
	int threshold;	   /* Changes less than this are ignored. */
	int x_factor;	   /* Factor used in computation with X-coordinates. */
	int x_sensors;	   /* The number of X-sensors. */
	int y_factor;	   /* Factor used in computation with Y-coordinates. */
	int y_sensors;	   /* The number of Y-sensors. */
	uint16_t product;  /* Product ID. */
	uint16_t vendor;   /* The vendor ID. */
};

/* Devices supported by this driver. */
static struct pbms_dev pbms_devices[] =
{
#define POWERBOOK_TOUCHPAD(inches, prod, x_fact, x_sens, y_fact)	      \
       {								      \
		.descr = #inches " inch PowerBook Trackpad",		      \
		.vendor = USB_VENDOR_APPLE,				      \
		.product = (prod),					      \
		.noise = 16,						      \
		.threshold = 5,						      \
		.x_factor = (x_fact),					      \
		.x_sensors = (x_sens),					      \
		.y_factor = (y_fact),					      \
		.y_sensors = 16						      \
       }
       /* 12 inch PowerBooks/iBooks */
       POWERBOOK_TOUCHPAD(12, 0x030a, 69, 16, 52), /* XXX Not tested. */
       POWERBOOK_TOUCHPAD(12, 0x030b, 73, 15, 96),
       /* 15 inch PowerBooks */
       POWERBOOK_TOUCHPAD(15, 0x020e, 85, 16, 57), /* XXX Not tested. */
       POWERBOOK_TOUCHPAD(15, 0x020f, 85, 16, 57),
       POWERBOOK_TOUCHPAD(15, 0x0215, 90, 15, 107),
       /* 17 inch PowerBooks */
       POWERBOOK_TOUCHPAD(17, 0x020d, 71, 26, 68)  /* XXX Not tested. */
#undef POWERBOOK_TOUCHPAD
};

/* The number of supported devices. */
#define PBMS_NUM_DEVICES (sizeof(pbms_devices) / sizeof(pbms_devices[0]))


/*
 * Types and prototypes.
 */


/* Device data. */
struct pbms_softc {
	struct uhidev sc_hdev;	      /* USB parent */
	int is_geyser2;
	int sc_datalen;		      /* Size of a data packet */
	int sc_bufusage;	      /* Number of bytes in sc_databuf */
	int sc_acc[PBMS_SENSORS];     /* Accumulated sensor values. */
	unsigned char sc_prev[PBMS_SENSORS];   /* Previous sample. */
	unsigned char sc_sample[PBMS_SENSORS]; /* Current sample. */
	uint8_t sc_databuf[PBMS_DATA_LEN];     /* Buffer for a data packet */
	device_t sc_wsmousedev; /* WSMouse device. */
	int sc_noise;		      /* Amount of noise. */
	int sc_theshold;	      /* Threshold value. */
	int sc_x;		      /* Virtual position in horizontal
				       * direction (wsmouse position). */
	int sc_x_factor;	      /* X-coordinate factor. */
	int sc_x_raw;		      /* X-position of finger on trackpad. */
	int sc_x_sensors;	      /* Number of X-sensors. */
	int sc_y;		      /* Virtual position in vertical direction
				       * (wsmouse position). */
	int sc_y_factor;	      /* Y-coordinate factor. */
	int sc_y_raw;		      /* Y-position of finger on trackpad. */
	int sc_y_sensors;	      /* Number of Y-sensors. */
	uint32_t sc_buttons;	      /* Button state. */
	uint32_t sc_status;	      /* Status flags. */
#define PBMS_ENABLED 1		      /* Is the device enabled? */
#define PBMS_DYING 2		      /* Is the device dying? */
#define PBMS_VALID 4		      /* Is the previous sample valid? */
};


/* Static function prototypes. */
static void pbms_intr(struct uhidev *, void *, unsigned int);
static int pbms_enable(void *);
static void pbms_disable(void *);
static int pbms_ioctl(void *, unsigned long, void *, int, struct lwp *);
static void reorder_sample(struct pbms_softc *, unsigned char *, unsigned char *);
static int compute_delta(struct pbms_softc *, int *, int *, int *, uint32_t *);
static int detect_pos(int *, int, int, int, int *, int *);
static int smooth_pos(int, int, int);

/* Access methods for wsmouse. */
const struct wsmouse_accessops pbms_accessops = {
	pbms_enable,
	pbms_ioctl,
	pbms_disable,
};

/* This take cares also of the basic device registration. */
int pbms_match(device_t, cfdata_t, void *);
void pbms_attach(device_t, device_t, void *);
int pbms_detach(device_t, int);
void pbms_childdet(device_t, device_t);
int pbms_activate(device_t, enum devact);
extern struct cfdriver pbms_cd;
CFATTACH_DECL2_NEW(pbms, sizeof(struct pbms_softc), pbms_match, pbms_attach,
    pbms_detach, pbms_activate, NULL, pbms_childdet);

/*
 * Basic driver.
 */


/* Try to match the device at some uhidev. */

int
pbms_match(device_t parent, cfdata_t match, void *aux)
{
	struct uhidev_attach_arg *uha = aux;
	usb_device_descriptor_t *udd;
	int i;
	uint16_t vendor, product;

	/*
	 * We just check if the vendor and product IDs have the magic numbers
	 * we expect.
	 */
	if (uha->uiaa->uiaa_proto == UIPROTO_MOUSE &&
	    ((udd = usbd_get_device_descriptor(uha->uiaa->uiaa_device))
		!= NULL)) {
		vendor = UGETW(udd->idVendor);
		product = UGETW(udd->idProduct);
		for (i = 0; i < PBMS_NUM_DEVICES; i++) {
			if (vendor == pbms_devices[i].vendor &&
			    product == pbms_devices[i].product)
				return UMATCH_IFACECLASS;
		}
	}
	return UMATCH_NONE;
}


/* Attach the device. */

void
pbms_attach(device_t parent, device_t self, void *aux)
{
	struct wsmousedev_attach_args a;
	struct uhidev_attach_arg *uha = aux;
	struct pbms_dev *pd;
	struct pbms_softc *sc = device_private(self);
	struct usbd_device *udev;
	usb_device_descriptor_t *udd;
	int i;
	uint16_t vendor, product;

	sc->sc_hdev.sc_intr = pbms_intr;
	sc->sc_hdev.sc_parent = uha->parent;
	sc->sc_hdev.sc_report_id = uha->reportid;

	sc->is_geyser2 = 0;
	sc->sc_datalen = PBMS_DATA_LEN;

	/* Fill in device-specific parameters. */
	udev = uha->uiaa->uiaa_udevice;
	if ((udd = usbd_get_device_descriptor(udev)) != NULL) {
		product = UGETW(udd->idProduct);
		vendor = UGETW(udd->idVendor);
		for (i = 0; i < PBMS_NUM_DEVICES; i++) {
			pd = &pbms_devices[i];
			if (product == pd->product && vendor == pd->vendor) {
				printf(": %s\n", pd->descr);
				sc->sc_noise = pd->noise;
				sc->sc_theshold = pd->threshold;
				sc->sc_x_factor = pd->x_factor;
				sc->sc_x_sensors = pd->x_sensors;
				sc->sc_y_factor = pd->y_factor;
				sc->sc_y_sensors = pd->y_sensors;
				if (product == 0x0215) {
					sc->is_geyser2 = 1;
					sc->sc_datalen = 64;
					sc->sc_y_sensors = 9;
				}
				else if (product == 0x030b)
					sc->sc_y_sensors = 9;
				break;
			}
		}
	}
	KASSERT(0 <= sc->sc_x_sensors && sc->sc_x_sensors <= PBMS_X_SENSORS);
	KASSERT(0 <= sc->sc_y_sensors && sc->sc_y_sensors <= PBMS_Y_SENSORS);

	sc->sc_status = 0;

	a.accessops = &pbms_accessops;
	a.accesscookie = sc;

	sc->sc_wsmousedev = config_found(self, &a, wsmousedevprint, CFARGS_NONE);

	return;
}


/* Detach the device. */

void
pbms_childdet(device_t self, device_t child)
{
	struct pbms_softc *sc = device_private(self);

	if (sc->sc_wsmousedev == child)
		sc->sc_wsmousedev = NULL;
}

int
pbms_detach(device_t self, int flags)
{
	/* XXX This could not possibly be sufficient! */
	return config_detach_children(self, flags);
}


/* Activate the device. */

int
pbms_activate(device_t self, enum devact act)
{
	struct pbms_softc *sc = device_private(self);

	if (act != DVACT_DEACTIVATE)
		return EOPNOTSUPP;

	sc->sc_status |= PBMS_DYING;
	return 0;
}


/* Enable the device. */

static int
pbms_enable(void *v)
{
	struct pbms_softc *sc = v;

	/* Check that we are not detaching or already enabled. */
	if (sc->sc_status & PBMS_DYING)
		return EIO;
	if (sc->sc_status & PBMS_ENABLED)
		return EBUSY;

	sc->sc_status |= PBMS_ENABLED;
	sc->sc_status &= ~PBMS_VALID;
	sc->sc_bufusage = 0;
	sc->sc_buttons = 0;
	memset(sc->sc_sample, 0, sizeof(sc->sc_sample));

	return uhidev_open(&sc->sc_hdev);
}


/* Disable the device. */

static void
pbms_disable(void *v)
{
	struct pbms_softc *sc = v;

	if (!(sc->sc_status & PBMS_ENABLED))
		return;

	sc->sc_status &= ~PBMS_ENABLED;
	uhidev_close(&sc->sc_hdev);
}


/* XXX ioctl not implemented. */

static int
pbms_ioctl(void *v, unsigned long cmd, void *data, int flag, struct lwp *p)
{
	return EPASSTHROUGH;
}


/*
 * Interrupts & pointer movement.
 */


/* Handle interrupts. */

void
pbms_intr(struct uhidev *addr, void *ibuf, unsigned int len)
{
	struct pbms_softc *sc = (struct pbms_softc *)addr;
	uint8_t *data;
	int dx, dy, dz, i, s;
	uint32_t buttons;

	/*
	 * We may have to construct the full data packet over two or three
	 * sequential interrupts, as the device only sends us chunks of
	 * 32 or 64 bytes of data.
	 * This also requires some synchronization, to make sure we place
	 * the first protocol-byte at the first byte in the bufffer.
	 */
	if (sc->is_geyser2) {
		/* XXX Need to check this. */
	} else {
		/* the last chunk is always 17 bytes */
		if (len == 17 && sc->sc_bufusage + len != sc->sc_datalen) {
			sc->sc_bufusage = 0;	/* discard bad packet */
			return;
		}
	}

	memcpy(sc->sc_databuf + sc->sc_bufusage, ibuf, len);
	sc->sc_bufusage += len;
	if (sc->sc_bufusage != sc->sc_datalen)
		return;		/* wait until packet is complete */

	/* process the now complete protocol and clear the buffer */
	data = sc->sc_databuf;
	sc->sc_bufusage = 0;
#if 0
	for (i = 0; i < sc->sc_datalen; i++)
		printf(" %02x", data[i]);
	printf("\n");
#endif

	/* The last byte is 1 if the button is pressed and 0 otherwise. */
	buttons = !!data[sc->sc_datalen - 1];

	/* Everything below assumes that the sample is reordered. */
	reorder_sample(sc, sc->sc_sample, data);

	/* Is this the first sample? */
	if (!(sc->sc_status & PBMS_VALID)) {
		sc->sc_status |= PBMS_VALID;
		sc->sc_x = sc->sc_y = -1;
		sc->sc_x_raw = sc->sc_y_raw = -1;
		memcpy(sc->sc_prev, sc->sc_sample, sizeof(sc->sc_prev));
		memset(sc->sc_acc, 0, sizeof(sc->sc_acc));
		return;
	}
	/* Accumulate the sensor change while keeping it nonnegative. */
	for (i = 0; i < PBMS_SENSORS; i++) {
		sc->sc_acc[i] +=
			(signed char) (sc->sc_sample[i] - sc->sc_prev[i]);
		if (sc->sc_acc[i] < 0)
			sc->sc_acc[i] = 0;
	}
	memcpy(sc->sc_prev, sc->sc_sample, sizeof(sc->sc_prev));

	/* Compute change. */
	dx = dy = dz = 0;
	if (!compute_delta(sc, &dx, &dy, &dz, &buttons))
		return;

	/* Report to wsmouse. */
	if ((dx != 0 || dy != 0 || dz != 0 || buttons != sc->sc_buttons) &&
	    sc->sc_wsmousedev != NULL) {
		s = spltty();
		wsmouse_input(sc->sc_wsmousedev, buttons, dx, -dy, dz, 0,
		    WSMOUSE_INPUT_DELTA);
		splx(s);
	}
	sc->sc_buttons = buttons;
}


/*
 * Reorder the sensor values so that all the X-sensors are before the
 * Y-sensors in the natural order. Note that this might have to be
 * rewritten if PBMS_X_SENSORS or PBMS_Y_SENSORS change.
 */

static void
reorder_sample(struct pbms_softc *sc, unsigned char *to, unsigned char *from)
{
	int i;

	if (sc->is_geyser2) {
		int j;

		memset(to, 0, PBMS_SENSORS);
		for (i = 0, j = 19; i < 20; i += 2, j += 3) {
			to[i] = from[j];
			to[i + 1] = from[j + 1];
		}
		for (i = 0, j = 1; i < 9; i += 2, j += 3) {
			to[PBMS_X_SENSORS + i] = from[j];
			to[PBMS_X_SENSORS + i + 1] = from[j + 1];
		}
	} else {
		for (i = 0; i < 8; i++) {
			/* X-sensors. */
			to[i] = from[5 * i + 2];
			to[i + 8] = from[5 * i + 4];
			to[i + 16] = from[5 * i + 42];
	#if 0
			/*
			 * XXX This seems to introduce random ventical jumps, so
			 * we ignore these sensors until we figure out their meaning.
			 */
			if (i < 2)
				to[i + 24] = from[5 * i + 44];
	#endif /* 0 */
			/* Y-sensors. */
			to[i + 26] = from[5 * i + 1];
			to[i + 34] = from[5 * i + 3];
		}
	}
}


/*
 * Compute the change in x, y and z direction, update the button state
 * (to simulate more than one button, scrolling etc.), and update the
 * history. Note that dx, dy, dz and buttons are modified only if
 * corresponding pressure is detected and should thus be initialised
 * before the call.  Return 0 on error.
 */

/* XXX Could we report something useful in dz? */

static int
compute_delta(struct pbms_softc *sc, int *dx, int *dy, int *dz,
	      uint32_t * buttons)
{
	int x_det, y_det, x_raw, y_raw, x_fingers, y_fingers, fingers, x, y;

	x_det = detect_pos(sc->sc_acc, sc->sc_x_sensors, sc->sc_theshold,
			   sc->sc_x_factor, &x_raw, &x_fingers);
	y_det = detect_pos(sc->sc_acc + PBMS_X_SENSORS, sc->sc_y_sensors,
			   sc->sc_theshold, sc->sc_y_factor,
			   &y_raw, &y_fingers);
	fingers = uimax(x_fingers, y_fingers);

	/* Check the number of fingers and if we have detected a position. */
	if (fingers > 1) {
		/* More than one finger detected, resetting. */
		memset(sc->sc_acc, 0, sizeof(sc->sc_acc));
		sc->sc_x_raw = sc->sc_y_raw = sc->sc_x = sc->sc_y = -1;
		return 0;
	} else if (x_det == 0 && y_det == 0) {
		/* No position detected, resetting. */
		memset(sc->sc_acc, 0, sizeof(sc->sc_acc));
		sc->sc_x_raw = sc->sc_y_raw = sc->sc_x = sc->sc_y = -1;
	} else if (x_det > 0 && y_det > 0) {
		/* Smooth position. */
		if (sc->sc_x_raw >= 0) {
			sc->sc_x_raw = (3 * sc->sc_x_raw + x_raw) / 4;
			sc->sc_y_raw = (3 * sc->sc_y_raw + y_raw) / 4;
			/*
			 * Compute virtual position and change if we already
			 * have a decent position.
			 */
			if (sc->sc_x >= 0) {
				x = smooth_pos(sc->sc_x, sc->sc_x_raw,
					       sc->sc_noise);
				y = smooth_pos(sc->sc_y, sc->sc_y_raw,
					       sc->sc_noise);
				*dx = x - sc->sc_x;
				*dy = y - sc->sc_y;
				sc->sc_x = x;
				sc->sc_y = y;
			} else {
				/* Initialise virtual position. */
				sc->sc_x = sc->sc_x_raw;
				sc->sc_y = sc->sc_y_raw;
			}
		} else {
			/* Initialise raw position. */
			sc->sc_x_raw = x_raw;
			sc->sc_y_raw = y_raw;
		}
	}
	return 1;
}


/*
 * Compute the new smoothed position from the previous smoothed position
 * and the raw position.
 */

static int
smooth_pos(int pos_old, int pos_raw, int noise)
{
	int ad, delta;

	delta = pos_raw - pos_old;
	ad = abs(delta);

	/* Too small changes are ignored. */
	if (ad < noise / 2)
		delta = 0;
	/* A bit larger changes are smoothed. */
	else if (ad < noise)
		delta /= 4;
	else if (ad < 2 * noise)
		delta /= 2;

	return pos_old + delta;
}


/*
 * Detect the position of the finger.  Returns the total pressure.
 * The position is returned in pos_ret and the number of fingers
 * is returned in fingers_ret.  The position returned in pos_ret
 * is in [0, (n_sensors - 1) * factor - 1].
 */

static int
detect_pos(int *sensors, int n_sensors, int threshold, int fact,
	   int *pos_ret, int *fingers_ret)
{
	int i, w, s;

	/*
	 * Compute the number of fingers, total pressure, and weighted
	 * position of the fingers.
	 */
	*fingers_ret = 0;
	w = s = 0;
	for (i = 0; i < n_sensors; i++) {
		if (sensors[i] >= threshold) {
			if (i == 0 || sensors[i - 1] < threshold)
				*fingers_ret += 1;
			s += sensors[i];
			w += sensors[i] * i;
		}
	}

	if (s > 0)
		*pos_ret = w * fact / s;

	return s;
}