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

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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>
This commit is contained in:
Thomas VanSelus 2017-12-22 10:51:23 -06:00 committed by Greg Kroah-Hartman
parent 2fa065fde2
commit 066ed42712
1 changed files with 18 additions and 1 deletions
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19
drivers/misc/ds1682.c
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@ -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 sensor_device_attribute_2 *sattr = to_sensor_dev_attr_2(attr);
struct i2c_client *client = to_i2c_client(dev); struct i2c_client *client = to_i2c_client(dev);
unsigned long long val; unsigned long long val, check;
__le32 val_le = 0; __le32 val_le = 0;
int rc; int rc;
@ -73,6 +73,23 @@ static ssize_t ds1682_show(struct device *dev, struct device_attribute *attr,
val = le32_to_cpu(val_le); 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 /* Format the output string and return # of bytes
* Special case: the 32 bit regs are time values with 1/4s * Special case: the 32 bit regs are time values with 1/4s
* resolution, scale them up to milliseconds * resolution, scale them up to milliseconds