i2c: slave-eeprom: add more info when to increase the pointer

It is a bit subtle when to correctly increase the buffer index when
reading. Make this clearer by adding some more comments and pointers to
the docs.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/i2c-slave-eeprom.c

@@ -48,12 +48,18 @@ static int i2c_slave_eeprom_slave_cb(struct i2c_client *client,
 		break;
 
 	case I2C_SLAVE_READ_PROCESSED:
+		/* The previous byte made it to the bus, get next one */
 		eeprom->buffer_idx++;
 		/* fallthrough */
 	case I2C_SLAVE_READ_REQUESTED:
 		spin_lock(&eeprom->buffer_lock);
 		*val = eeprom->buffer[eeprom->buffer_idx];
 		spin_unlock(&eeprom->buffer_lock);
+		/*
+		 * Do not increment buffer_idx here, because we don't know if
+		 * this byte will be actually used. Read Linux I2C slave docs
+		 * for details.
+		 */
 		break;
 
 	case I2C_SLAVE_STOP: