net: dsa: microchip: ksz8_r_sta_mac_table(): Avoid using error code for empty entries

Prepare for the next patch by ensuring that ksz8_r_sta_mac_table() does
not use error codes for empty entries. This change will enable better
handling of read/write errors in the upcoming patch.
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/dsa/microchip/ksz8795.c

@@ -458,7 +458,7 @@ int ksz8_r_dyn_mac_table(struct ksz_device *dev, u16 addr, u8 *mac_addr,
 }
 
 static int ksz8_r_sta_mac_table(struct ksz_device *dev, u16 addr,
-				struct alu_struct *alu)
+				struct alu_struct *alu, bool *valid)
 {
 	u32 data_hi, data_lo;
 	const u8 *shifts;
@@ -471,28 +471,32 @@ static int ksz8_r_sta_mac_table(struct ksz_device *dev, u16 addr,
 	ksz8_r_table(dev, TABLE_STATIC_MAC, addr, &data);
 	data_hi = data >> 32;
 	data_lo = (u32)data;
-	if (data_hi & (masks[STATIC_MAC_TABLE_VALID] |
-			masks[STATIC_MAC_TABLE_OVERRIDE])) {
-		alu->mac[5] = (u8)data_lo;
-		alu->mac[4] = (u8)(data_lo >> 8);
-		alu->mac[3] = (u8)(data_lo >> 16);
-		alu->mac[2] = (u8)(data_lo >> 24);
-		alu->mac[1] = (u8)data_hi;
-		alu->mac[0] = (u8)(data_hi >> 8);
-		alu->port_forward =
-			(data_hi & masks[STATIC_MAC_TABLE_FWD_PORTS]) >>
-				shifts[STATIC_MAC_FWD_PORTS];
-		alu->is_override =
-			(data_hi & masks[STATIC_MAC_TABLE_OVERRIDE]) ? 1 : 0;
-		data_hi >>= 1;
-		alu->is_static = true;
-		alu->is_use_fid =
-			(data_hi & masks[STATIC_MAC_TABLE_USE_FID]) ? 1 : 0;
-		alu->fid = (data_hi & masks[STATIC_MAC_TABLE_FID]) >>
-				shifts[STATIC_MAC_FID];
+
+	if (!(data_hi & (masks[STATIC_MAC_TABLE_VALID] |
+			 masks[STATIC_MAC_TABLE_OVERRIDE]))) {
+		*valid = false;
 		return 0;
 	}
-	return -ENXIO;
+
+	alu->mac[5] = (u8)data_lo;
+	alu->mac[4] = (u8)(data_lo >> 8);
+	alu->mac[3] = (u8)(data_lo >> 16);
+	alu->mac[2] = (u8)(data_lo >> 24);
+	alu->mac[1] = (u8)data_hi;
+	alu->mac[0] = (u8)(data_hi >> 8);
+	alu->port_forward =
+		(data_hi & masks[STATIC_MAC_TABLE_FWD_PORTS]) >>
+			shifts[STATIC_MAC_FWD_PORTS];
+	alu->is_override = (data_hi & masks[STATIC_MAC_TABLE_OVERRIDE]) ? 1 : 0;
+	data_hi >>= 1;
+	alu->is_static = true;
+	alu->is_use_fid = (data_hi & masks[STATIC_MAC_TABLE_USE_FID]) ? 1 : 0;
+	alu->fid = (data_hi & masks[STATIC_MAC_TABLE_FID]) >>
+		shifts[STATIC_MAC_FID];
+
+	*valid = true;
+
+	return 0;
 }
 
 void ksz8_w_sta_mac_table(struct ksz_device *dev, u16 addr,
@@ -981,20 +985,25 @@ static int ksz8_add_sta_mac(struct ksz_device *dev, int port,
 			    const unsigned char *addr, u16 vid)
 {
 	struct alu_struct alu;
-	int index;
+	int index, ret;
 	int empty = 0;
 
 	alu.port_forward = 0;
 	for (index = 0; index < dev->info->num_statics; index++) {
-		if (!ksz8_r_sta_mac_table(dev, index, &alu)) {
-			/* Found one already in static MAC table. */
-			if (!memcmp(alu.mac, addr, ETH_ALEN) &&
-			    alu.fid == vid)
-				break;
-		/* Remember the first empty entry. */
-		} else if (!empty) {
-			empty = index + 1;
+		bool valid;
+
+		ret = ksz8_r_sta_mac_table(dev, index, &alu, &valid);
+		if (ret)
+			return ret;
+		if (!valid) {
+			/* Remember the first empty entry. */
+			if (!empty)
+				empty = index + 1;
+			continue;
 		}
+
+		if (!memcmp(alu.mac, addr, ETH_ALEN) && alu.fid == vid)
+			break;
 	}
 
 	/* no available entry */
@@ -1024,15 +1033,19 @@ static int ksz8_del_sta_mac(struct ksz_device *dev, int port,
 			    const unsigned char *addr, u16 vid)
 {
 	struct alu_struct alu;
-	int index;
+	int index, ret;
 
 	for (index = 0; index < dev->info->num_statics; index++) {
-		if (!ksz8_r_sta_mac_table(dev, index, &alu)) {
-			/* Found one already in static MAC table. */
-			if (!memcmp(alu.mac, addr, ETH_ALEN) &&
-			    alu.fid == vid)
-				break;
-		}
+		bool valid;
+
+		ret = ksz8_r_sta_mac_table(dev, index, &alu, &valid);
+		if (ret)
+			return ret;
+		if (!valid)
+			continue;
+
+		if (!memcmp(alu.mac, addr, ETH_ALEN) && alu.fid == vid)
+			break;
 	}
 
 	/* no available entry */