NFC: trf7970a: Ignore antenna noise when checking for RF field

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>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Link: https://patch.msgid.link/20260422100930.581237-1-paul.geurts@prodrive-technologies.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/nfc/trf7970a.c b/drivers/nfc/trf7970a.c
index d17c701c7888b3be0b46cf95c4b77182313c97c0..08c27bb438b59713b77aa9c4073868d6fb30752e 100644 (file)
--- 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;