Re: [PATCH 2/2] hwmon: max31827: Functional enhancement of the driver

From: Guenter Roeck
Date: Tue Sep 12 2023 - 14:43:52 EST

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On Tue, Sep 12, 2023 at 07:58:51AM +0000, Matyas, Daniel wrote:
>
>
> > -----Original Message-----
> > From: Guenter Roeck <groeck7@xxxxxxxxx> On Behalf Of Guenter
> > Roeck
> > Sent: Monday, September 11, 2023 4:46 PM
> > To: Matyas, Daniel <Daniel.Matyas@xxxxxxxxxx>
> > Cc: Jean Delvare <jdelvare@xxxxxxxx>; Rob Herring
> > <robh+dt@xxxxxxxxxx>; Jonathan Corbet <corbet@xxxxxxx>; linux-
> > hwmon@xxxxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; linux-doc@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH 2/2] hwmon: max31827: Functional enhancement of
> > the driver
> >
> > [External]
> >
> > On 9/11/23 01:37, Daniel Matyas wrote:
> > > Modify some bits depending on the device tree.
> > >
> >
> > The subject and the above sentence are meaningless. Please improve.
> >
> > > Added custom device attributes for resolution and timeout. The wait
> > > time for a conversion in one-shot mode (enable = 0) depends on the
> > > resolution.
> > >
> > > Used enums and static const arrays in order to remove nested switch
> > > blocks.
> > >
> >
> > One logical change per patch, please. Do not mix no-trivial cleanups with
> > functional changes.
> >
> > > Signed-off-by: Daniel Matyas <daniel.matyas@xxxxxxxxxx>
> > > ---
> > > Documentation/hwmon/max31827.rst | 65 ++++--
> > > drivers/hwmon/max31827.c | 326
> > ++++++++++++++++++++++++-------
> > > 2 files changed, 306 insertions(+), 85 deletions(-)
> > >
> > > diff --git a/Documentation/hwmon/max31827.rst
> > > b/Documentation/hwmon/max31827.rst
> > > index b0971d05b8a4..ae884e9e6085 100644
> > > --- a/Documentation/hwmon/max31827.rst
> > > +++ b/Documentation/hwmon/max31827.rst
> > > @@ -52,13 +52,21 @@ MAX31827 has low and over temperature
> > alarms with an effective value and a
> > > hysteresis value: -40 and -30 degrees for under temperature alarm
> > and +100 and
> > > +90 degrees for over temperature alarm.
> > >
> > > -The alarm can be configured in comparator and interrupt mode.
> > > Currently only -comparator mode is implemented. In Comparator
> > mode,
> > > the OT/UT status bits have a -value of 1 when the temperature rises
> > > above the TH value or falls below TL, -which is also subject to the
> > > Fault Queue selection. OT status returns to 0 when -the temperature
> > drops below the TH_HYST value or when shutdown mode is entered.
> > > -Similarly, UT status returns to 0 when the temperature rises above
> > > TL_HYST value -or when shutdown mode is entered.
> > > +The alarm can be configured in comparator and interrupt mode from
> > the
> > > +devicetree. In Comparator mode, the OT/UT status bits have a value
> > of
> > > +1 when the temperature rises above the TH value or falls below TL,
> > > +which is also subject to the Fault Queue selection. OT status returns
> > > +to 0 when the temperature drops below the TH_HYST value or when
> > > +shutdown mode is entered. Similarly, UT status returns to 0 when the
> > > +temperature rises above TL_HYST value or when shutdown mode is
> > entered.
> > > +
> > > +In interrupt mode exceeding TH also sets OT status to 1, which
> > > +remains set until a read operation is performed on the
> > > +configuration/status register (max or min attribute); at this point,
> > > +it returns to 0. Once OT status is set to 1 from exceeding TH and
> > > +reset, it is set to 1 again only when the temperature drops below
> > > +TH_HYST. The output remains asserted until it is reset by a read. It
> > > +is set again if the temperature rises above TH, and so on. The same
> > logic applies to the operation of the UT status bit.
> > >
> > > Putting the MAX31827 into shutdown mode also resets the OT/UT
> > status bits. Note
> > > that if the mode is changed while OT/UT status bits are set, an
> > > OT/UT status @@ -68,13 +76,32 @@ clear the status bits before
> > changing the operating mode.
> > >
> > > The conversions can be manual with the one-shot functionality and
> > automatic with
> > > a set frequency. When powered on, the chip measures temperatures
> > with 1 conv/s.
> > > +The conversion rate can be modified with update_interval attribute of
> > the chip.
> > > +Conversion/second = 1/update_interval. Thus, the available options
> > > +according to the data sheet are:
> > > + - 64000 (ms) = 1 conv/64 sec
> > > + - 32000 (ms) = 1 conv/32 sec
> > > + - 16000 (ms) = 1 conv/16 sec
> > > + - 4000 (ms) = 1 conv/4 sec
> > > + - 1000 (ms) = 1 conv/sec (default)
> > > + - 250 (ms) = 4 conv/sec
> > > + - 125 (ms) = 8 conv/sec
> > > +
> > > Enabling the device when it is already enabled has the side effect of
> > setting
> > > the conversion frequency to 1 conv/s. The conversion time varies
> > > depending on -the resolution. The conversion time doubles with every
> > > bit of increased -resolution. For 10 bit resolution 35ms are needed,
> > > while for 12 bit resolution
> > > -(default) 140ms. When chip is in shutdown mode and a read operation
> > > is -requested, one-shot is triggered, the device waits for 140
> > > (conversion time) + 1
> > > -(error) ms, and only after that is the temperature value register read.
> > > +the resolution.
> > > +
> > > +The conversion time doubles with every bit of increased resolution.
> > > +The available resolutions are:
> > > + - 8 bit -> 8.75 ms conversion time
> > > + - 9 bit -> 17.5 ms conversion time
> > > + - 10 bit -> 35 ms conversion time
> > > + - 12 bit (default) -> 140 ms conversion time
> > > +
> > > +When chip is in shutdown mode and a read operation is requested,
> > > +one-shot is triggered, the device waits for <conversion time> ms, and
> > > +only after that is the temperature value register read. Note that the
> > > +conversion times are rounded up to the nearest possible integer.
> > >
> > > The LSB of the temperature values is 0.0625 degrees Celsius, but the
> > values of
> > > the temperatures are displayed in milli-degrees. This means, that
> > > some data is @@ -83,8 +110,18 @@ in the writing of alarm values too.
> > For positive numbers the user-input value
> > > will always be rounded down to the nearest possible value, for
> > negative numbers
> > > the user-input will always be rounded up to the nearest possible
> > value.
> > >
> > > +Bus timeout resets the I2C-compatible interface when SCL is low for
> > > +more than 30ms (nominal).
> > > +
> > > +Alarm polarity determines if the active state of the alarm is low or
> > > +high. The behavior for both settings is dependent on the Fault Queue
> > > +setting. The ALARM pin is an open-drain output and requires a pullup
> > resistor to operate.
> > > +
> > > +The Fault Queue bits select how many consecutive temperature
> > faults
> > > +must occur before overtemperature or undertemperature faults are
> > > +indicated in the corresponding status bits.
> > > +
> > > Notes
> > > -----
> > >
> > > -Currently fault queue, alarm polarity and resolution cannot be
> > modified.
> > > -PEC is not implemented either.
> > > +PEC is not implemented.
> > > diff --git a/drivers/hwmon/max31827.c b/drivers/hwmon/max31827.c
> > index
> > > 66f5fcf937ff..9f48dec99b7f 100644
> > > --- a/drivers/hwmon/max31827.c
> > > +++ b/drivers/hwmon/max31827.c
> > > @@ -12,6 +12,18 @@
> > > #include <linux/i2c.h>
> > > #include <linux/mutex.h>
> > > #include <linux/regmap.h>
> > > +#include <linux/hwmon-sysfs.h>
> > > +
> > > +/*
> > > + * gcc turns __builtin_ffsll() into a call to __ffsdi2(), which is
> > > +not provided
> > > + * by every architecture. __ffs64() is available on all
> > > +architectures, but the
> > > + * result is not defined if no bits are set.
> > > + */
> > > +#define max31827__bf_shf(x) \
> > > + ({ \
> > > + typeof(x) x_ = (x); \
> > > + ((x_) != 0) ? __ffs64(x_) : 0x0; \
> > > + })
> > >
> > > #define MAX31827_T_REG 0x0
> > > #define MAX31827_CONFIGURATION_REG 0x2
> > > @@ -22,22 +34,120 @@
> > >
> > > #define MAX31827_CONFIGURATION_1SHOT_MASK BIT(0)
> > > #define MAX31827_CONFIGURATION_CNV_RATE_MASK
> > GENMASK(3, 1)
> > > +#define MAX31827_CONFIGURATION_TIMEOUT_MASK BIT(5)
> > > +#define MAX31827_CONFIGURATION_RESOLUTION_MASK
> > GENMASK(7, 6)
> > > +#define MAX31827_CONFIGURATION_ALRM_POL_MASK BIT(8)
> > > +#define MAX31827_CONFIGURATION_COMP_INT_MASK BIT(9)
> > > +#define MAX31827_CONFIGURATION_FLT_Q_MASK
> > GENMASK(11, 10)
> > > #define MAX31827_CONFIGURATION_U_TEMP_STAT_MASK BIT(14)
> > > #define MAX31827_CONFIGURATION_O_TEMP_STAT_MASK BIT(15)
> > >
> > > -#define MAX31827_12_BIT_CNV_TIME 141
> > > -
> > > -#define MAX31827_CNV_1_DIV_64_HZ 0x1
> > > -#define MAX31827_CNV_1_DIV_32_HZ 0x2
> > > -#define MAX31827_CNV_1_DIV_16_HZ 0x3
> > > -#define MAX31827_CNV_1_DIV_4_HZ 0x4
> > > -#define MAX31827_CNV_1_HZ 0x5
> > > -#define MAX31827_CNV_4_HZ 0x6
> > > -#define MAX31827_CNV_8_HZ 0x7
> > > +#define MAX31827_8_BIT_CNV_TIME 9
> > > +#define MAX31827_9_BIT_CNV_TIME 18
> > > +#define MAX31827_10_BIT_CNV_TIME 35
> > > +#define MAX31827_12_BIT_CNV_TIME 140
> > >
> > > #define MAX31827_16_BIT_TO_M_DGR(x) (sign_extend32(x, 15) *
> > 1000 / 16)
> > > #define MAX31827_M_DGR_TO_16_BIT(x) (((x) << 4) / 1000)
> > > #define MAX31827_DEVICE_ENABLE(x) ((x) ? 0xA : 0x0)
> > > +#define MAX31827_FLT_Q
> > > +
> > > +enum max31827_cnv {
> > > + MAX31827_CNV_1_DIV_64_HZ = 1,
> > > + MAX31827_CNV_1_DIV_32_HZ,
> > > + MAX31827_CNV_1_DIV_16_HZ,
> > > + MAX31827_CNV_1_DIV_4_HZ,
> > > + MAX31827_CNV_1_HZ,
> > > + MAX31827_CNV_4_HZ,
> > > + MAX31827_CNV_8_HZ,
> > > +};
> > > +
> > > +static const u16 max31827_conversions[] = {
> > > + [MAX31827_CNV_1_DIV_64_HZ] = 64000,
> > > + [MAX31827_CNV_1_DIV_32_HZ] = 32000,
> > > + [MAX31827_CNV_1_DIV_16_HZ] = 16000,
> > > + [MAX31827_CNV_1_DIV_4_HZ] = 4000,
> > > + [MAX31827_CNV_1_HZ] = 1000,
> > > + [MAX31827_CNV_4_HZ] = 250,
> > > + [MAX31827_CNV_8_HZ] = 125,
> > > +};
> > > +
> > > +enum max31827_resolution {
> > > + MAX31827_RES_8_BIT = 0,
> > > + MAX31827_RES_9_BIT,
> > > + MAX31827_RES_10_BIT,
> > > + MAX31827_RES_12_BIT,
> > > +};
> > > +
> > > +static const u16 max31827_resolutions[] = {
> > > + [MAX31827_RES_8_BIT] = 8,
> > > + [MAX31827_RES_9_BIT] = 9,
> > > + [MAX31827_RES_10_BIT] = 10,
> > > + [MAX31827_RES_12_BIT] = 12,
> > > +};
> > > +
> > > +enum cfg_index {
> > > + cfg_timeout_idx = 0,
> > > + cfg_res_idx,
> > > + cfg_num
> > > +};
> > > +
> > > +static const unsigned int cfg_masks[cfg_num] = {
> > > + [cfg_timeout_idx] =
> > MAX31827_CONFIGURATION_TIMEOUT_MASK,
> > > + [cfg_res_idx] =
> > MAX31827_CONFIGURATION_RESOLUTION_MASK,
> > > +};
> > > +
> > > +enum max31827_cnv {
> > > + MAX31827_CNV_1_DIV_64_HZ = 1,
> > > + MAX31827_CNV_1_DIV_32_HZ,
> > > + MAX31827_CNV_1_DIV_16_HZ,
> > > + MAX31827_CNV_1_DIV_4_HZ,
> > > + MAX31827_CNV_1_HZ,
> > > + MAX31827_CNV_4_HZ,
> > > + MAX31827_CNV_8_HZ,
> > > +};
> > > +
> > > +static const u16 max31827_conversions[] = {
> > > + [MAX31827_CNV_1_DIV_64_HZ] = 64000,
> > > + [MAX31827_CNV_1_DIV_32_HZ] = 32000,
> > > + [MAX31827_CNV_1_DIV_16_HZ] = 16000,
> > > + [MAX31827_CNV_1_DIV_4_HZ] = 4000,
> > > + [MAX31827_CNV_1_HZ] = 1000,
> > > + [MAX31827_CNV_4_HZ] = 250,
> > > + [MAX31827_CNV_8_HZ] = 125,
> > > +};
> > > +
> > > +enum max31827_resolution {
> > > + MAX31827_RES_8_BIT = 0,
> > > + MAX31827_RES_9_BIT,
> > > + MAX31827_RES_10_BIT,
> > > + MAX31827_RES_12_BIT,
> > > +};
> > > +
> > > +static const u16 max31827_resolutions[] = {
> > > + [MAX31827_RES_8_BIT] = 8,
> > > + [MAX31827_RES_9_BIT] = 9,
> > > + [MAX31827_RES_10_BIT] = 10,
> > > + [MAX31827_RES_12_BIT] = 12,
> > > +};
> > > +
> > > +static const u16 max31827_conv_times[] = {
> > > + [MAX31827_RES_8_BIT] = MAX31827_8_BIT_CNV_TIME,
> > > + [MAX31827_RES_9_BIT] = MAX31827_9_BIT_CNV_TIME,
> > > + [MAX31827_RES_10_BIT] = MAX31827_10_BIT_CNV_TIME,
> > > + [MAX31827_RES_12_BIT] = MAX31827_12_BIT_CNV_TIME, };
> > > +
> > > +enum cfg_index {
> > > + cfg_timeout_idx = 0,
> > > + cfg_res_idx,
> > > + cfg_num
> > > +};
> > > +
> > > +static const unsigned int cfg_masks[cfg_num] = {
> > > + [cfg_timeout_idx] =
> > MAX31827_CONFIGURATION_TIMEOUT_MASK,
> > > + [cfg_res_idx] =
> > MAX31827_CONFIGURATION_RESOLUTION_MASK,
> > > +};
> > >
> > > struct max31827_state {
> > > /*
> > > @@ -46,6 +156,7 @@ struct max31827_state {
> > > struct mutex lock;
> > > struct regmap *regmap;
> > > bool enable;
> > > + unsigned int resolution;
> > > };
> > >
> > > static const struct regmap_config max31827_regmap = { @@ -166,8
> > > +277,7 @@ static int max31827_read(struct device *dev, enum
> > hwmon_sensor_types type,
> > > mutex_unlock(&st->lock);
> > > return ret;
> > > }
> > > -
> > > - msleep(MAX31827_12_BIT_CNV_TIME);
> > > + msleep(max31827_conv_times[st-
> > >resolution]);
> > > }
> > > ret = regmap_read(st->regmap,
> > MAX31827_T_REG, &uval);
> > >
> > > @@ -243,32 +353,7 @@ static int max31827_read(struct device *dev,
> > enum
> > > hwmon_sensor_types type,
> > >
> > > uval =
> > FIELD_GET(MAX31827_CONFIGURATION_CNV_RATE_MASK,
> > > uval);
> > > - switch (uval) {
> > > - case MAX31827_CNV_1_DIV_64_HZ:
> > > - *val = 64000;
> > > - break;
> > > - case MAX31827_CNV_1_DIV_32_HZ:
> > > - *val = 32000;
> > > - break;
> > > - case MAX31827_CNV_1_DIV_16_HZ:
> > > - *val = 16000;
> > > - break;
> > > - case MAX31827_CNV_1_DIV_4_HZ:
> > > - *val = 4000;
> > > - break;
> > > - case MAX31827_CNV_1_HZ:
> > > - *val = 1000;
> > > - break;
> > > - case MAX31827_CNV_4_HZ:
> > > - *val = 250;
> > > - break;
> > > - case MAX31827_CNV_8_HZ:
> > > - *val = 125;
> > > - break;
> > > - default:
> > > - *val = 0;
> > > - break;
> > > - }
> > > + *val = max31827_conversions[uval];
> > > }
> > > break;
> > >
> > > @@ -284,6 +369,7 @@ static int max31827_write(struct device *dev,
> > enum hwmon_sensor_types type,
> > > u32 attr, int channel, long val)
> > > {
> > > struct max31827_state *st = dev_get_drvdata(dev);
> > > + int res = 1;
> > > int ret;
> > >
> > > switch (type) {
> > > @@ -333,39 +419,27 @@ static int max31827_write(struct device *dev,
> > enum hwmon_sensor_types type,
> > > if (!st->enable)
> > > return -EOPNOTSUPP;
> > >
> > > - switch (val) {
> > > - case 125:
> > > - val = MAX31827_CNV_8_HZ;
> > > - break;
> > > - case 250:
> > > - val = MAX31827_CNV_4_HZ;
> > > - break;
> > > - case 1000:
> > > - val = MAX31827_CNV_1_HZ;
> > > - break;
> > > - case 4000:
> > > - val = MAX31827_CNV_1_DIV_4_HZ;
> > > - break;
> > > - case 16000:
> > > - val = MAX31827_CNV_1_DIV_16_HZ;
> > > - break;
> > > - case 32000:
> > > - val = MAX31827_CNV_1_DIV_32_HZ;
> > > - break;
> > > - case 64000:
> > > - val = MAX31827_CNV_1_DIV_64_HZ;
> > > - break;
> > > - default:
> > > + /*
> > > + * Convert the desired conversion rate into
> > register
> > > + * bits. res is already initialized with 1.
> > > + *
> > > + * This was inspired by lm73 driver.
> > > + */
> > > + while (res < ARRAY_SIZE(max31827_conversions)
> > &&
> > > + val < max31827_conversions[res])
> > > + res++;
> > > +
> > > + if (res == ARRAY_SIZE(max31827_conversions) ||
> > > + val != max31827_conversions[res])
> > > return -EOPNOTSUPP;
> > > - }
> > >
> > > - val =
> > FIELD_PREP(MAX31827_CONFIGURATION_CNV_RATE_MASK,
> > > - val);
> > > + res =
> > FIELD_PREP(MAX31827_CONFIGURATION_CNV_RATE_MASK,
> > > + res);
> > >
> > > return regmap_update_bits(st->regmap,
> > >
> > MAX31827_CONFIGURATION_REG,
> > >
> > MAX31827_CONFIGURATION_CNV_RATE_MASK,
> > > - val);
> > > + res);
> > > }
> > > break;
> > >
> > > @@ -376,14 +450,121 @@ static int max31827_write(struct device
> > *dev, enum hwmon_sensor_types type,
> > > return -EOPNOTSUPP;
> > > }
> > >
> > > -static int max31827_init_client(struct max31827_state *st)
> > > +static ssize_t cfg_show(struct device *dev, struct device_attribute
> > *devattr,
> > > + char *buf)
> > > {
> > > + struct sensor_device_attribute *attr =
> > to_sensor_dev_attr(devattr);
> > > + struct max31827_state *st = dev_get_drvdata(dev);
> > > + const unsigned int mask = cfg_masks[attr->index];
> > > + unsigned int val, res;
> > > + int ret;
> > > +
> > > + ret = regmap_read(st->regmap,
> > MAX31827_CONFIGURATION_REG, &val);
> > > + /*
> > > + * FIELD_GET did not work because mask is not a compile time
> > constant.
> > > + * There is no bitfield check now.
> > > + */
> > > + val = (val & mask) >> max31827__bf_shf(mask);
> > > +
> > > + if (attr->index == cfg_res_idx)
> > > + res = max31827_resolutions[val];
> > > + else
> > > + res = !val;
> > > +
> > > + return scnprintf(buf, PAGE_SIZE, "%u\n", res); }
> > > +
> > > +static ssize_t cfg_store(struct device *dev, struct device_attribute
> > *devattr,
> > > + const char *buf, size_t count)
> > > +{
> > > + struct sensor_device_attribute *attr =
> > to_sensor_dev_attr(devattr);
> > > + struct max31827_state *st = dev_get_drvdata(dev);
> > > + const unsigned int mask = cfg_masks[attr->index];
> > > + unsigned int res = 0;
> > > + unsigned int val;
> > > + int ret;
> > > +
> > > + ret = kstrtouint(buf, 10, &val);
> > > + if (ret)
> > > + return ret;
> > > +
> > > + if (attr->index == cfg_res_idx) {
> > > + /*
> > > + * Convert the desired conversion rate into register
> > > + * bits. res is already initialized with 1.
> > > + *
> > > + * This was inspired by lm73 driver.
> > > + */
> > > + while (res < ARRAY_SIZE(max31827_resolutions) &&
> > > + val > max31827_resolutions[res])
> > > + res++;
> > > +
> > > + if (res == ARRAY_SIZE(max31827_resolutions) ||
> > > + val != max31827_resolutions[res])
> > > + return -EOPNOTSUPP;
> > > +
> > > + st->resolution = res;
> > > + } else {
> > > + res = !val;
> > > + }
> > > +
> > > + /*
> > > + * FIELD_PREP did not work because mask is not a compile time
> > constant.
> > > + * There is no bitfield check now.
> > > + */
> > > + res = (res << max31827__bf_shf(mask)) & mask;
> > > + ret = regmap_update_bits(st->regmap,
> > MAX31827_CONFIGURATION_REG,
> > > + cfg_masks[attr->index], res);
> > > +
> > > + return (ret) ? ret : count;
> > > +}
> > > +
> > > +static SENSOR_DEVICE_ATTR_RW(cfg_resolution, cfg, cfg_res_idx);
> > > +static SENSOR_DEVICE_ATTR_RW(cfg_timeout, cfg, cfg_timeout_idx);
> > > +
> > > +static struct attribute *max31827_attrs[] = {
> > > + &sensor_dev_attr_cfg_resolution.dev_attr.attr,
> > > + &sensor_dev_attr_cfg_timeout.dev_attr.attr,
> >
> >
> > Those attributes are not acceptable. First of all, the cfg_ prefix is useless.
> > The timeout is system related and should be a devicetree property.
> > The resolution maps into the number of conversions per second, and
> > therefore the standard update_interval property can be used.
> >
> > Guenter
> >
>
> The cfg_ prefix is used, because both timeout and resolution can be modified
> from the configuration register. I can set timeout to be a devicetree property,
> then I would not need the cfg_ prefix.
>

The name of a sysfs attribute has nothing to do with how it is interpreted.
Those prefixes are not acceptable.

> However, I am not sure about setting the resolution mapping into the update
> interval. Only the 12-bit resolution's nominal conversion time (140 ms) and the
> 8 conv/sec conversion rate (update interval = 125 ms) are comparable (this is
> something that I recently observed, and I will handle this case in the next patch).
> In the rest of cases, the conversion time is much smaller, than the update interval,
> thus, it can be neglected.
>
> The conversion time induced by the resolution is only relevant, when the device is
> disabled and a read is performed. In this case, one-shot is triggered, the driver sleeps
> for the time needed to do the conversion, and finally does the read of the temperature
> data.
>
> In fact, you were the one who suggested using a sysfs attribute for resolution, instead
> of update interval:
> https://marc.info/?l=linux-i2c&m=169201795715687&w=2
>

Good point. But then I also said

"ec related attributes should be attached to the i2c interface. Other
drivers already do that; look for pec_{show,store}. The same is true
^^^^^^^^^^^^^^^^
for timeout."
^^^^^^^^^^^

Resolution as sysfs attribute would have to use a well defined unit.
Just using a number doesn't help, since "number of bits" may or may not
include the sign and doesn't say how many of those bits reflect full
degrees C. I'd be inclined to accept a "resolution" attribute if
specified in milli-degrees C. This would have to have a temp_ prefix
since, after all, we could have something similar for other sensor
types.

Guenter

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