[PATCH] input: ads7846: correct the value got from SPI

[Igor Grinberg <grinberg@compulab.co.il>]

From: Andrey Gelman <andrey.gelman@compulab.co.il>

According to the touch controller spec, SPI return a 16 bit value,
only 12 bits are valid, they are bit[14-3].

The value of MISO and MOSI can be configured when SPI is in idle mode.
Currently this touch driver assumes the SPI bus sets the MOSI and
MISO in low level when SPI bus is in idle mode. So the bit[15] of the
value got from SPI bus is always 0. But when SPI bus congfigures the
MOSI and MISO in high level during the SPI idle mode, the bit[15] of
the value get from SPI is always 1. If bit[15] is not masked, we may
get the wrong value.

Mask the invalid bit to make sure the correct value gets returned.
Regardless of the SPI bus idle configuration.

Signed-off-by: Andrey Gelman <andrey.gelman@compulab.co.il>
Signed-off-by: Haibo Chen <haibo.chen@freescale.com>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
---
 drivers/input/touchscreen/ads7846.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
index 727d88c..8e2a44d 100644
--- a/drivers/input/touchscreen/ads7846.c
+++ b/drivers/input/touchscreen/ads7846.c
@@ -666,18 +666,22 @@ static int ads7846_no_filter(void *ads, int data_idx, int *val)
 
 static int ads7846_get_value(struct ads7846 *ts, struct spi_message *m)
 {
+	int value;
 	struct spi_transfer *t =
 		list_entry(m->transfers.prev, struct spi_transfer, transfer_list);
 
 	if (ts->model == 7845) {
-		return be16_to_cpup((__be16 *)&(((char*)t->rx_buf)[1])) >> 3;
+		value = be16_to_cpup((__be16 *)&(((char*)t->rx_buf)[1]));
 	} else {
 		/*
 		 * adjust:  on-wire is a must-ignore bit, a BE12 value, then
 		 * padding; built from two 8 bit values written msb-first.
 		 */
-		return be16_to_cpup((__be16 *)t->rx_buf) >> 3;
+		value = be16_to_cpup((__be16 *)t->rx_buf);
 	}
+
+	/* enforce ADC output is 12 bits width */
+	return (value >> 3) & 0xfff;
 }
 
 static void ads7846_update_value(struct spi_message *m, int val)
-- 
2.4.9