[PATCH] NFC: trf7970a: Ignore antenna noise when checking for RF field

[Paul Geurts <paul.geurts@prodrive-technologies.com>]

The main channel Received Signal Strength Indicator (RSSI) measurement
is used to determine whether an RF field is present or not. RSSI != 0
is interpreted as an RF Field is present. This does not take RF noise
and measurement inaccuracy into account, and results in false positives
in the field.

Define a noise level and make sure the RF field is only interpreted as
present when the RSSI is above the noise level.

Fixes: 851ee3cbf850 ("NFC: trf7970a: Don't turn on RF if there is already an RF field")
Signed-off-by: Paul Geurts <paul.geurts@prodrive-technologies.com>
---
 drivers/nfc/trf7970a.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/nfc/trf7970a.c b/drivers/nfc/trf7970a.c
index d17c701c7888..08c27bb438b5 100644
--- a/drivers/nfc/trf7970a.c
+++ b/drivers/nfc/trf7970a.c
@@ -317,6 +317,7 @@
 #define TRF7970A_RSSI_OSC_STATUS_RSSI_MASK	(BIT(2) | BIT(1) | BIT(0))
 #define TRF7970A_RSSI_OSC_STATUS_RSSI_X_MASK	(BIT(5) | BIT(4) | BIT(3))
 #define TRF7970A_RSSI_OSC_STATUS_RSSI_OSC_OK	BIT(6)
+#define TRF7970A_RSSI_OSC_STATUS_RSSI_NOISE_LEVEL	1
 
 #define TRF7970A_SPECIAL_FCN_REG1_COL_7_6		BIT(0)
 #define TRF7970A_SPECIAL_FCN_REG1_14_ANTICOLL		BIT(1)
@@ -1300,7 +1301,7 @@ static int trf7970a_is_rf_field(struct trf7970a *trf, bool *is_rf_field)
 	if (ret)
 		return ret;
 
-	if (rssi & TRF7970A_RSSI_OSC_STATUS_RSSI_MASK)
+	if ((rssi & TRF7970A_RSSI_OSC_STATUS_RSSI_MASK) > TRF7970A_RSSI_OSC_STATUS_RSSI_NOISE_LEVEL)
 		*is_rf_field = true;
 	else
 		*is_rf_field = false;
-- 
2.39.2