Input: resistive-adc-touch - add support for z1 and z2 channels

This patch adds support for the z1 and z2 channels. These are used to
calculate the applied pressure. As there is no common order of the
individual channels of a resistive touch ADC, support for
io-channel-names is added (although the DT bindings stated the driver
already supports these).
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20210525054634.9134-5-o.rempel@pengutronix.deSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/touchscreen/resistive-adc-touch.c

@@ -20,7 +20,18 @@
 
 #define DRIVER_NAME					"resistive-adc-touch"
 #define GRTS_DEFAULT_PRESSURE_MIN			50000
+#define GRTS_DEFAULT_PRESSURE_MAX			65535
 #define GRTS_MAX_POS_MASK				GENMASK(11, 0)
+#define GRTS_MAX_CHANNELS				4
+
+enum grts_ch_type {
+	GRTS_CH_NONE = 0,
+	GRTS_CH_X,
+	GRTS_CH_Y,
+	GRTS_CH_PRESSURE,
+	GRTS_CH_Z1,
+	GRTS_CH_Z2,
+};
 
 /**
  * struct grts_state - generic resistive touch screen information struct
@@ -33,24 +44,59 @@
  */
 struct grts_state {
 	u32				pressure_min;
+	u32				x_plate_ohms;
 	bool				pressure;
 	struct iio_channel		*iio_chans;
 	struct iio_cb_buffer		*iio_cb;
 	struct input_dev		*input;
 	struct touchscreen_properties	prop;
+	u8				ch[GRTS_MAX_CHANNELS];
 };
 
 static int grts_cb(const void *data, void *private)
 {
 	const u16 *touch_info = data;
 	struct grts_state *st = private;
-	unsigned int x, y, press = 0x0;
+	unsigned int x, y, press = 0, z1 = 0, z2;
+	unsigned int Rt, i;
+
+	for (i = 0; i < ARRAY_SIZE(st->ch) && st->ch[i] != GRTS_CH_NONE; i++) {
+		switch (st->ch[i]) {
+		case GRTS_CH_X:
+			x = touch_info[i];
+			break;
+		case GRTS_CH_Y:
+			y = touch_info[i];
+			break;
+		case GRTS_CH_PRESSURE:
+			press = touch_info[i];
+			break;
+		case GRTS_CH_Z1:
+			z1 = touch_info[i];
+			break;
+		case GRTS_CH_Z2:
+			z2 = touch_info[i];
+			break;
+		}
+	}
 
-	/* channel data coming in buffer in the order below */
-	x = touch_info[0];
-	y = touch_info[1];
-	if (st->pressure)
-		press = touch_info[2];
+	if (z1) {
+		Rt = z2;
+		Rt -= z1;
+		Rt *= st->x_plate_ohms;
+		Rt = DIV_ROUND_CLOSEST(Rt, 16);
+		Rt *= x;
+		Rt /= z1;
+		Rt = DIV_ROUND_CLOSEST(Rt, 256);
+		/*
+		 * On increased pressure the resistance (Rt) is decreasing
+		 * so, convert values to make it looks as real pressure.
+		 */
+		if (Rt < GRTS_DEFAULT_PRESSURE_MAX)
+			press = GRTS_DEFAULT_PRESSURE_MAX - Rt;
+		else
+			press = 0;
+	}
 
 	if ((!x && !y) || (st->pressure && (press < st->pressure_min))) {
 		/* report end of touch */
@@ -94,12 +140,77 @@ static void grts_disable(void *data)
 	iio_channel_release_all_cb(data);
 }
 
+static int grts_get_properties(struct grts_state *st, struct device *dev)
+{
+	int idx, error;
+
+	idx = device_property_match_string(dev, "io-channel-names", "x");
+	if (idx < 0)
+		return idx;
+
+	if (idx >= ARRAY_SIZE(st->ch))
+		return -EOVERFLOW;
+
+	st->ch[idx] = GRTS_CH_X;
+
+	idx = device_property_match_string(dev, "io-channel-names", "y");
+	if (idx < 0)
+		return idx;
+
+	if (idx >= ARRAY_SIZE(st->ch))
+		return -EOVERFLOW;
+
+	st->ch[idx] = GRTS_CH_Y;
+
+	/* pressure is optional */
+	idx = device_property_match_string(dev, "io-channel-names", "pressure");
+	if (idx >= 0) {
+		if (idx >= ARRAY_SIZE(st->ch))
+			return -EOVERFLOW;
+
+		st->ch[idx] = GRTS_CH_PRESSURE;
+		st->pressure = true;
+
+		return 0;
+	}
+
+	/* if no pressure is defined, try optional z1 + z2 */
+	idx = device_property_match_string(dev, "io-channel-names", "z1");
+	if (idx < 0)
+		return 0;
+
+	if (idx >= ARRAY_SIZE(st->ch))
+		return -EOVERFLOW;
+
+	st->ch[idx] = GRTS_CH_Z1;
+
+	/* if z1 is provided z2 is not optional */
+	idx = device_property_match_string(dev, "io-channel-names", "z2");
+	if (idx < 0)
+		return idx;
+
+	if (idx >= ARRAY_SIZE(st->ch))
+		return -EOVERFLOW;
+
+	st->ch[idx] = GRTS_CH_Z2;
+	st->pressure = true;
+
+	error = device_property_read_u32(dev,
+					 "touchscreen-x-plate-ohms",
+					 &st->x_plate_ohms);
+	if (error) {
+		dev_err(dev, "can't get touchscreen-x-plate-ohms property\n");
+		return error;
+	}
+
+	return 0;
+}
+
 static int grts_probe(struct platform_device *pdev)
 {
 	struct grts_state *st;
 	struct input_dev *input;
 	struct device *dev = &pdev->dev;
-	struct iio_channel *chan;
 	int error;
 
 	st = devm_kzalloc(dev, sizeof(struct grts_state), GFP_KERNEL);
@@ -115,12 +226,13 @@ static int grts_probe(struct platform_device *pdev)
 		return error;
 	}
 
-	chan = &st->iio_chans[0];
-	st->pressure = false;
-	while (chan && chan->indio_dev) {
-		if (!strcmp(chan->channel->datasheet_name, "pressure"))
-			st->pressure = true;
-		chan++;
+	if (!device_property_present(dev, "io-channel-names"))
+		return -ENODEV;
+
+	error = grts_get_properties(st, dev);
+	if (error) {
+		dev_err(dev, "Failed to parse properties\n");
+		return error;
 	}
 
 	if (st->pressure) {
@@ -148,7 +260,7 @@ static int grts_probe(struct platform_device *pdev)
 	input_set_abs_params(input, ABS_Y, 0, GRTS_MAX_POS_MASK - 1, 0, 0);
 	if (st->pressure)
 		input_set_abs_params(input, ABS_PRESSURE, st->pressure_min,
-				     0xffff, 0, 0);
+				     GRTS_DEFAULT_PRESSURE_MAX, 0, 0);
 
 	input_set_capability(input, EV_KEY, BTN_TOUCH);