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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