NFC: trf7970a: Only write 'ISO Control' when its changing

The current code always writes to the 'ISO Control' register when the
RF framing is set.  That's not necessary since the register's value
doesn't always change. Instead, only write to it when its value is
actually being changed.
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/nfc/trf7970a.c

@@ -337,6 +337,7 @@ struct trf7970a {
 	nfc_digital_cmd_complete_t	cb;
 	void				*cb_arg;
 	u8				iso_ctrl;
+	u8				iso_ctrl_tech;
 	u8				special_fcn_reg1;
 	int				technology;
 	int				framing;
@@ -787,6 +788,7 @@ static int trf7970a_init(struct trf7970a *trf)
 	if (ret)
 		goto err_out;
 
+	trf->iso_ctrl = 0xff;
 	return 0;
 
 err_out:
@@ -889,10 +891,10 @@ static int trf7970a_config_rf_tech(struct trf7970a *trf, int tech)
 
 	switch (tech) {
 	case NFC_DIGITAL_RF_TECH_106A:
-		trf->iso_ctrl = TRF7970A_ISO_CTRL_14443A_106;
+		trf->iso_ctrl_tech = TRF7970A_ISO_CTRL_14443A_106;
 		break;
 	case NFC_DIGITAL_RF_TECH_ISO15693:
-		trf->iso_ctrl = TRF7970A_ISO_CTRL_15693_SGL_1OF4_2648;
+		trf->iso_ctrl_tech = TRF7970A_ISO_CTRL_15693_SGL_1OF4_2648;
 		break;
 	default:
 		dev_dbg(trf->dev, "Unsupported rf technology: %d\n", tech);
@@ -906,24 +908,27 @@ static int trf7970a_config_rf_tech(struct trf7970a *trf, int tech)
 
 static int trf7970a_config_framing(struct trf7970a *trf, int framing)
 {
+	u8 iso_ctrl = trf->iso_ctrl_tech;
+	int ret;
+
 	dev_dbg(trf->dev, "framing: %d\n", framing);
 
 	switch (framing) {
 	case NFC_DIGITAL_FRAMING_NFCA_SHORT:
 	case NFC_DIGITAL_FRAMING_NFCA_STANDARD:
 		trf->tx_cmd = TRF7970A_CMD_TRANSMIT_NO_CRC;
-		trf->iso_ctrl |= TRF7970A_ISO_CTRL_RX_CRC_N;
+		iso_ctrl |= TRF7970A_ISO_CTRL_RX_CRC_N;
 		break;
 	case NFC_DIGITAL_FRAMING_NFCA_STANDARD_WITH_CRC_A:
 	case NFC_DIGITAL_FRAMING_NFCA_T4T:
 	case NFC_DIGITAL_FRAMING_ISO15693_INVENTORY:
 	case NFC_DIGITAL_FRAMING_ISO15693_T5T:
 		trf->tx_cmd = TRF7970A_CMD_TRANSMIT;
-		trf->iso_ctrl &= ~TRF7970A_ISO_CTRL_RX_CRC_N;
+		iso_ctrl &= ~TRF7970A_ISO_CTRL_RX_CRC_N;
 		break;
 	case NFC_DIGITAL_FRAMING_NFCA_T2T:
 		trf->tx_cmd = TRF7970A_CMD_TRANSMIT;
-		trf->iso_ctrl |= TRF7970A_ISO_CTRL_RX_CRC_N;
+		iso_ctrl |= TRF7970A_ISO_CTRL_RX_CRC_N;
 		break;
 	default:
 		dev_dbg(trf->dev, "Unsupported Framing: %d\n", framing);
@@ -932,7 +937,15 @@ static int trf7970a_config_framing(struct trf7970a *trf, int framing)
 
 	trf->framing = framing;
 
-	return trf7970a_write(trf, TRF7970A_ISO_CTRL, trf->iso_ctrl);
+	if (iso_ctrl != trf->iso_ctrl) {
+		ret = trf7970a_write(trf, TRF7970A_ISO_CTRL, iso_ctrl);
+		if (ret)
+			return ret;
+
+		trf->iso_ctrl = iso_ctrl;
+	}
+
+	return 0;
 }
 
 static int trf7970a_in_configure_hw(struct nfc_digital_dev *ddev, int type,