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(-) >> > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-iio" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
