Re: [PATCH] hwmon: (tmp102) Force wait for conversion time for the first valid data

[Nishanth Menon <nm@ti.com>]

On 07:47-20151201, Nishanth Menon wrote:
> Hi Guenter,
> 
> Thanks for the detailed review..
> 
> On 11/30/2015 11:50 PM, Guenter Roeck wrote:
> > On 11/30/2015 08:25 PM, Nishanth Menon wrote:
> [...]
> 
> >>
> >> A simpler alternative approach could be to sleep in the probe for the
> >> duration required, but that will result in latency that is undesirable
> >> that can delay boot sequence un-necessarily.
> >>
> > A really simpler solution would be to mark when the device is ready
> > to be accessed in the probe function, and go to sleep for the remaining
> > time
> > in the update function if necessary. This would not affect the probe
> > function,
> > avoid the somewhat awkward -EAGAIN, avoid overloading the value cache,
> > and only
> > sleep if necessary and as long as needed.
> 
> We already have that logic in a different form:
> We use last_update to know when to go read the temperature value. Until
> the conversion time has elapsed, we keep providing previously cached
> value. Trouble is the first time read before conversion time is complete:
> 
> On sleep during update:
> unfortunately, forcing the delay in update for the first time:
> a) Will also cause the latency in the thermal_zone_device_check which
> triggers right after tmp102_probe->thermal_zone_of_sensor_register
> b) -EAGAIN is used by other hwmon drivers such as
> drivers/hwmon/adt7470.c, drivers/hwmon/ltc4245.c, drivers/hwmon/sht15.c,
> drivers/hwmon/tc74.c, drivers/hwmon/via-cputemp.c in similar ways when
> data cannot be provided back.
> 
> Overriding the temp value to indicate first time read:
> I can setup a bool in struct tmp102 instead -> but that serves the same
> purpose as what we did with override, except increase 1 char footprint -
> though I agree, it might be a little more readable.
> 
> > 
> >> [1] http://www.ti.com/lit/ds/symlink/tmp102.pdf
> >>
> >> Cc: Eduardo Valentin <edubezval@gmail.com>
> >> Reported-by: Aparna Balasubramanian <aparnab@ti.com>
> >> Reported-by: Elvita Lobo <elvita@ti.com>
> >> Reported-by: Yan Liu <yan-liu@ti.com>
> >> Signed-off-by: Nishanth Menon <nm@ti.com>
> >> ---
> >>
> >> Example case (from Beagleboard-x15 using an older kernel revision):
> >>     http://pastebin.ubuntu.com/13591711/
> >> Notice the thermal shutdown trigger:
> >>     thermal thermal_zone3: critical temperature reached(108
> >> C),shutting down
> >>
> >>   drivers/hwmon/tmp102.c | 19 ++++++++++++++++++-
> >>   1 file changed, 18 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/hwmon/tmp102.c b/drivers/hwmon/tmp102.c
> >> index 65482624ea2c..145f69108f23 100644
> >> --- a/drivers/hwmon/tmp102.c
> >> +++ b/drivers/hwmon/tmp102.c
> >> @@ -50,6 +50,9 @@
> >>   #define    TMP102_TLOW_REG            0x02
> >>   #define    TMP102_THIGH_REG        0x03
> >>
> >> +/* TMP102 range is -55 to 150C -> we use -128 as a default invalid
> >> value */
> >> +#define TMP102_NOTREADY            -128
> >> +
> > 
> > This is a bit misleading, and also not correct, since the temperature is
> > stored in
> > milli-degrees C, so a value of -128 reflects -0.128 degreees C. While
> > that value
> > will not be seen in practice, it is still not a good idea to use it for
> > this purpose.
> > 
> > Even though the chip temperature range is -55 .. 150 C, that doesn't mean
> > it never returns a value outside that range, for example if nothing is
> > connected
> > to an external sensor or if something is broken.
> > 
> > You should use a value outside the value range, ie outside
> > [-128,000 .. 127,999 ] to detect the "not ready" condition.
> 
> 
> That is true.. I will just drop this and introduce a bool in tmp102 instead.
> 
> >>   struct tmp102 {
> >>       struct i2c_client *client;
> >>       struct device *hwmon_dev;
> >> @@ -102,6 +105,12 @@ static int tmp102_read_temp(void *dev, int *temp)
> >>   {
> >>       struct tmp102 *tmp102 = tmp102_update_device(dev);
> >>
> >> +    /* Is it too early even to return a conversion? */
> >> +    if (tmp102->temp[0] == TMP102_NOTREADY) {
> >> +        dev_dbg(dev, "%s: Conversion not ready yet..\n", __func__);
> >> +        return -EAGAIN;
> > 
> > Does this cause a hard loop in the calling code, or will the thermal code
> > delay before it reads again ?
> > 
> > If it causes a hard loop, it may be better to go to sleep if needed
> > when reading the data, as suggested above.
> 
> Thermal framework is capable of handling -EAGAIN without a hard loop
> around this (it just seems to reschedule around the polling interval and
> comes back to check if data is ready).
> 
> If you are ok with the above, then I will send a v2 introducing a bool
> to setup a flag for first_time read, but will leave the -EAGAIN alone.

Hint about how the patch will look like:
diff --git a/drivers/hwmon/tmp102.c b/drivers/hwmon/tmp102.c
index 65482624ea2c..5289aa0980a8 100644
--- a/drivers/hwmon/tmp102.c
+++ b/drivers/hwmon/tmp102.c
@@ -58,6 +58,7 @@ struct tmp102 {
 	u16 config_orig;
 	unsigned long last_update;
 	int temp[3];
+	bool first_time;
 };
 
 /* convert left adjusted 13-bit TMP102 register value to milliCelsius */
@@ -93,6 +94,7 @@ static struct tmp102 *tmp102_update_device(struct device *dev)
 				tmp102->temp[i] = tmp102_reg_to_mC(status);
 		}
 		tmp102->last_update = jiffies;
+		tmp102->first_time = false;
 	}
 	mutex_unlock(&tmp102->lock);
 	return tmp102;
@@ -102,6 +104,12 @@ static int tmp102_read_temp(void *dev, int *temp)
 {
 	struct tmp102 *tmp102 = tmp102_update_device(dev);
 
+	/* Is it too early even to return a conversion? */
+	if (tmp102->first_time) {
+		dev_dbg(dev, "%s: Conversion not ready yet..\n", __func__);
+		return -EAGAIN;
+	}
+
 	*temp = tmp102->temp[0];
 
 	return 0;
@@ -114,6 +122,10 @@ static ssize_t tmp102_show_temp(struct device *dev,
 	struct sensor_device_attribute *sda = to_sensor_dev_attr(attr);
 	struct tmp102 *tmp102 = tmp102_update_device(dev);
 
+	/* Is it too early even to return a read? */
+	if (tmp102->first_time)
+		return -EAGAIN;
+
 	return sprintf(buf, "%d\n", tmp102->temp[sda->index]);
 }
 
@@ -207,7 +219,9 @@ static int tmp102_probe(struct i2c_client *client,
 		status = -ENODEV;
 		goto fail_restore_config;
 	}
-	tmp102->last_update = jiffies - HZ;
+	tmp102->last_update = jiffies;
+	/* Mark that we are not ready with data until conversion is complete */
+	tmp102->first_time = true;
 	mutex_init(&tmp102->lock);
 
 	hwmon_dev = hwmon_device_register_with_groups(dev, client->name,
-- 
Regards,
Nishanth Menon