rtc: rv8803: workaround i2c HW issue

The rv8803 has a 60µs window where it will not answer on the i2c bus.
It also means there will be no ack for the communication. Make sure
communication is tried multiple times when this happens (the i2c subsystem
mandates -ENXIO is that case but the number of retries is host specific).
The critical parts are the probe function and the alarm callback so make
sure we handle the failure there.

Cc: stable@vger.kernel.org # v4.4
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-rv8803.c

@@ -61,11 +61,14 @@ static irqreturn_t rv8803_handle_irq(int irq, void *dev_id)
 	struct i2c_client *client = dev_id;
 	struct rv8803_data *rv8803 = i2c_get_clientdata(client);
 	unsigned long events = 0;
-	int flags;
+	int flags, try = 0;
 
 	mutex_lock(&rv8803->flags_lock);
 
-	flags = i2c_smbus_read_byte_data(client, RV8803_FLAG);
+	do {
+		flags = i2c_smbus_read_byte_data(client, RV8803_FLAG);
+		try++;
+	} while ((flags == -ENXIO) && (try < 3));
 	if (flags <= 0) {
 		mutex_unlock(&rv8803->flags_lock);
 		return IRQ_NONE;
@@ -424,7 +427,7 @@ static int rv8803_probe(struct i2c_client *client,
 {
 	struct i2c_adapter *adapter = to_i2c_adapter(client->dev.parent);
 	struct rv8803_data *rv8803;
-	int err, flags;
+	int err, flags, try = 0;
 
 	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |
 				     I2C_FUNC_SMBUS_I2C_BLOCK)) {
@@ -441,7 +444,16 @@ static int rv8803_probe(struct i2c_client *client,
 	rv8803->client = client;
 	i2c_set_clientdata(client, rv8803);
 
-	flags = i2c_smbus_read_byte_data(client, RV8803_FLAG);
+	/*
+	 * There is a 60µs window where the RTC may not reply on the i2c bus in
+	 * that case, the transfer is not ACKed. In that case, ensure there are
+	 * multiple attempts.
+	 */
+	do {
+		flags = i2c_smbus_read_byte_data(client, RV8803_FLAG);
+		try++;
+	} while ((flags == -ENXIO) && (try < 3));
+
 	if (flags < 0)
 		return flags;
 
@@ -476,8 +488,12 @@ static int rv8803_probe(struct i2c_client *client,
 		return PTR_ERR(rv8803->rtc);
 	}
 
-	err = i2c_smbus_write_byte_data(rv8803->client, RV8803_EXT,
-					RV8803_EXT_WADA);
+	try = 0;
+	do {
+		err = i2c_smbus_write_byte_data(rv8803->client, RV8803_EXT,
+						RV8803_EXT_WADA);
+		try++;
+	} while ((err == -ENXIO) && (try < 3));
 	if (err)
 		return err;