Re: [PATCH 0/2] Fix erroneous pressure scaling for the lps331ap driver

[Jonathan Cameron <jic23@kernel.org>]

Denis,

Any comments on this series?

(Jacek, I'm not entirely sure if you managed to add Denis as intended
in your last email - certainly no sign of his email when it got to me.)

On 07/02/2013 12:06 PM, Jacek Anaszewski wrote:
> Adding Denis.
> 
> On 07/02/2013 12:13 PM, Jacek Anaszewski wrote:
>> This series fixes pressure scale value calculation and modifies the
>> way how the temperature scale value is calculated, for clarity
>> and consistency reasons. Below the detailed explanation
>> for the fix is provided.
>>
>> Because of the IIO_VAL_INT_PLUS_NANO value returned by the
>> read_raw callback for the IIO_CHAN_INFO_SCALE mask case
>> the returned scale value has to be expressed in nanopascal
>> units. The scale value is used then for obtaining pressure
>> value in kilopascal units. Currently this value is 2441410,
>> whereas it should be 24414.
>>
>> 1. Scaling of exemplary read_raw value:
>>
>> exemplary read raw result: 4067471
>> buggy_scale: 0.002441410 =>  2441410 * 10^-9
>> buggy_pressure: 4067471 * 0.002441410 =
>> 9930.4
>> According to the documentation file
>> Documentation/ABI/testing/sysfs-bus-iio the scaled pressure
>> value is expressed in kilopascals, so the result should
>> be interpreted as 9930.4 kPa
>>
>> 2. Scaling of the device output value according to the data sheet
>>
>> In order to get the result in mbars according to the device
>> data sheet the output value has to be divided by 4096 (LSB/mbar)
>>
>> pressure [mbar] = 4067471 / 4096 = 993.0349
>>
>> Given that 1 bar = 10^5 Pa the result in pascal units is
>> 993.0349 * 10^-3 * 10^5 Pa = 993.0349 * 10^2 Pa = 993 hPa =>
>> 99.3 kPa
>>
>> whereas 4067471 * 0.002441410 (the buggy_scale value) = 9930.4,
>> which when considered as a result in kilopascals is not true
>> according to the above reasoning.
>>
>> 3. Proposed fix
>>
>> The proposed fix introduces following constants:
>>
>> ST_PRESS_LSB_PER_MBAR (4096.0) - LSB/mbar
>>
>> ST_PRESS_KPASCAL_NANO_SCALE (100000000 / ST_PRESS_LSB_PER_MBAR) -
>> actual scale factor to be returned by in_pressure_scale which after
>> performing division gives the value 24414.
>> in_pressure_scale returns 0.000024414 after applying nano scale.
>>
>> The final result in kilopascal units can be obtained as follows:
>>
>> in_pressure_raw * in_pressure_scale =
>> = 4067471 * 24414 * 10^-9 = 4.067471 * 10^6 * 2.4414 * 10^4 * 10^-9 =
>> = 9.930 * 10^1 = 99.3 kPa =>  993 hPa (same as the pressure result
>> obtained in the paragraph 2.)
>>
>> Summarizing, the bug was in the ST_PRESS_MBAR_TO_KPASCAL macro
>> which performed multiplication instead of division.
>>
>> The second patch applies similar operations for the temperature scale
>> calculaton. Previous implementation was correct but the modification
>> is made for consistency and clarity.
>>
>> Thanks,
>> Jacek
>>
>> Jacek Anaszewski (2):
>>    iio: lps331ap: Fix wrong in_pressure_scale output value
>>    iio: lps331ap: Modify in_temp_scale calculation way
>>
>>   drivers/iio/pressure/st_pressure_core.c |   11 ++++++++---
>>   1 files changed, 8 insertions(+), 3 deletions(-)
>>
> 
> 
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