iio: mxs-lradc: change the realbits to 12

The LRADC virtual channels have an 18 bit field to store the sum of up
to 2^5 accumulated samples. The read_raw function however only operates
over a single sample (12 bit resolution).
In order to use this field for scaling operations, we need it to be the
exact resolution value of the LRADC.
Besides, the driver was using an 18 bit mask (LRADC_CH_VALUE_MASK) to
report touch coordinates to userland. A 12 bit mask should be used instead
or else the touch libraries will expect a coordinates range between 0
and 0x3ffff (18 bits), instead of between 0 and 0xfff (12 bits).

Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/mxs-lradc.c

@@ -225,6 +225,9 @@ struct mxs_lradc {
 #define	LRADC_CTRL4_LRADCSELECT_MASK(n)		(0xf << ((n) * 4))
 #define	LRADC_CTRL4_LRADCSELECT_OFFSET(n)	((n) * 4)
 
+#define LRADC_RESOLUTION			12
+#define LRADC_SINGLE_SAMPLE_MASK		((1 << LRADC_RESOLUTION) - 1)
+
 /*
  * Raw I/O operations
  */
@@ -540,9 +543,10 @@ static int mxs_lradc_ts_register(struct mxs_lradc *lradc)
 	__set_bit(EV_ABS, input->evbit);
 	__set_bit(EV_KEY, input->evbit);
 	__set_bit(BTN_TOUCH, input->keybit);
-	input_set_abs_params(input, ABS_X, 0, LRADC_CH_VALUE_MASK, 0, 0);
-	input_set_abs_params(input, ABS_Y, 0, LRADC_CH_VALUE_MASK, 0, 0);
-	input_set_abs_params(input, ABS_PRESSURE, 0, LRADC_CH_VALUE_MASK, 0, 0);
+	input_set_abs_params(input, ABS_X, 0, LRADC_SINGLE_SAMPLE_MASK, 0, 0);
+	input_set_abs_params(input, ABS_Y, 0, LRADC_SINGLE_SAMPLE_MASK, 0, 0);
+	input_set_abs_params(input, ABS_PRESSURE, 0, LRADC_SINGLE_SAMPLE_MASK,
+			     0, 0);
 
 	lradc->ts_input = input;
 	input_set_drvdata(input, lradc);
@@ -817,7 +821,7 @@ static const struct iio_buffer_setup_ops mxs_lradc_buffer_ops = {
 	.channel = (idx),					\
 	.scan_type = {						\
 		.sign = 'u',					\
-		.realbits = 18,					\
+		.realbits = LRADC_RESOLUTION,			\
 		.storagebits = 32,				\
 	},							\
 }