misc: ds1682: Ignore update-in-progress ETC reads

The Elapsed Time Counter (ETC) registers are not buffered for reading.
If a 250ms tick occurs while data is being read out, the result can be
a combination of old and new values. This can occur at the byte level
(giving a time in the future) or the individual bit level (giving a
time in the past). We catch both these cases by reading until we get
two equal or consecutive values. After five unsuccessful attempts we
give up.

Signed-off-by: Thomas VanSelus <tvanselus@xes-inc.com>
Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/misc/ds1682.c b/drivers/misc/ds1682.c
index 0fc5366..98a921e 100644 (file)
--- a/drivers/misc/ds1682.c
+++ b/drivers/misc/ds1682.c
@@ -59,7 +59,7 @@ static ssize_t ds1682_show(struct device *dev, struct device_attribute *attr,
 {
        struct sensor_device_attribute_2 *sattr = to_sensor_dev_attr_2(attr);
        struct i2c_client *client = to_i2c_client(dev);
-       unsigned long long val;
+       unsigned long long val, check;
        __le32 val_le = 0;
        int rc;
 
@@ -73,6 +73,23 @@ static ssize_t ds1682_show(struct device *dev, struct device_attribute *attr,
 
        val = le32_to_cpu(val_le);
 
+       if (sattr->index == DS1682_REG_ELAPSED) {
+               int retries = 5;
+
+               /* Detect and retry when a tick occurs mid-read */
+               do {
+                       rc = i2c_smbus_read_i2c_block_data(client, sattr->index,
+                                                          sattr->nr,
+                                                          (u8 *)&val_le);
+                       if (rc < 0 || retries <= 0)
+                               return -EIO;
+
+                       check = val;
+                       val = le32_to_cpu(val_le);
+                       retries--;
+               } while (val != check && val != (check + 1));
+       }
+
        /* Format the output string and return # of bytes
         * Special case: the 32 bit regs are time values with 1/4s
         * resolution, scale them up to milliseconds