On 05/26/2015 06:00 PM, Guenter Roeck wrote:
On Tue, May 26, 2015 at 03:41:42PM -0500, Chris Lesiak wrote:
The function ntc_adc_iio_read was using iio_read_channel_raw and
converting to microVolts using the assumption of a 12 bit ADC. If a
converter with different precision was used, the result was in error.
Instead, use iio_read_channel_processed, which does the conversion to
microVolts itself.
Signed-off-by: Chris Lesiak <chris.lesiak@xxxxxxxxx>
---
drivers/hwmon/ntc_thermistor.c | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)
diff --git a/drivers/hwmon/ntc_thermistor.c b/drivers/hwmon/ntc_thermistor.c
index 6880011..0d3a686 100644
--- a/drivers/hwmon/ntc_thermistor.c
+++ b/drivers/hwmon/ntc_thermistor.c
@@ -190,20 +190,15 @@ struct ntc_data {
static int ntc_adc_iio_read(struct ntc_thermistor_platform_data *pdata)
{
struct iio_channel *channel = pdata->chan;
- s64 result;
int val, ret;
- ret = iio_read_channel_raw(channel, &val);
+ ret = iio_read_channel_processed(channel, &val);
if (ret < 0) {
pr_err("read channel() error: %d\n", ret);
return ret;
}
- /* unit: mV */
- result = pdata->pullup_uv * (s64) val;
- result >>= 12;
-
- return (int)result;
+ return val;
The calculation used to need pullup_uv. As far as I can see,
the iio driver knows nothing about pullup_uv, which makes me
wonder how this can work. Can you explain ?
Thanks,
Guenter
On further review, it would be premature to apply that patch.
Although it is good circuit design practice for pullup_uv to be the same
as the ADC vref, that is not always the case. The ntc_thermistor driver
currently assumes they are the same and that the ADC is 12 bit.
24 of the 31 iio/adc device drivers in the 4.0 tree know their vref,
either from device tree or pdata (example at91_adc.c), via
regulator_get_voltage (example mcp320x.c), as a fixed internal reference
(example max1027.c), or a combination of the above (example ad7266.c).
The other 7 do not support IIO_CHAN_INFO_SCALE. They are:
axp288_adc.c
exynos_adc.c
men_z188_adc.c
ti_am335x_adc.c
twl4030-madc.c
twl6030-gpadc.c
viperboard_adc.c
A look at the arm device trees show that ntc_thermistor is used with
exynos_adc on both exynos5420-peach-pit and exynos5800-peach-pi. This
patch would break those platforms.
One solution would be to add support for IIO_CHAN_INFO_SCALE to the rest
of the adc drivers, or at least exynos_adc.
I'll submit a patch taking a different approach: use
iio_read_channel_processed only when supported by the adc.
--
Chris Lesiak
Principal Design Engineer, Software
LI-COR, Inc.
chris.lesiak@xxxxxxxxx
Any opinions expressed are those of the author and
do not necessarily represent those of his employer.
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