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thermal: imx: improve comments describing algorithm for temp calculation

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The description of the implemented algorithm is hardly understandable
without having the right application note side-by-side to the code.

Fix this by using shorter and more intuitive variable names, describe
their meaning and transform a single formula instead of first talking about
slope and then about "milli_Tmeas".

There are no code changes.

Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
This commit is contained in:
Uwe Kleine-König 2017-11-30 10:17:36 +01:00 committed by Eduardo Valentin
parent e4bb2240d4
commit c5bbdb4ba3
1 changed files with 19 additions and 23 deletions
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42
drivers/thermal/imx_thermal.c
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@ -359,32 +359,28 @@ static int imx_init_calib(struct platform_device *pdev, u32 ocotp_ana1)
} }
/* /*
* Sensor data layout: * The sensor is calibrated at 25 °C (aka T1) and the value measured
* [31:20] - sensor value @ 25C * (aka N1) at this temperature is provided in bits [31:20] in the
* Use universal formula now and only need sensor value @ 25C * i.MX's OCOTP value ANA1.
* slope = 0.4297157 - (0.0015976 * 25C fuse) * To find the actual temperature T, the following formula has to be used
* when reading value n from the sensor:
*
* T = T1 + (N - N1) / (0.4297157 - 0.0015976 * N1) °C
* = [T1 - N1 / (0.4297157 - 0.0015976 * N1) °C] + N / (0.4297157 - 0.0015976 * N1) °C
* = [T1 + N1 / (0.0015976 * N1 - 0.4297157) °C] - N / (0.0015976 * N1 - 0.4297157) °C
* = c2 - c1 * N
*
* with
*
* c1 = 1 / (0.0015976 * N1 - 0.4297157) °C
* c2 = T1 + N1 / (0.0015976 * N1 - 0.4297157) °C
* = T1 + N1 * C1
*/ */
n1 = ocotp_ana1 >> 20; n1 = ocotp_ana1 >> 20;
t1 = 25; /* t1 always 25C */ t1 = 25; /* °C */
/* temp64 = FACTOR0; /* 10^7 for FACTOR1 and FACTOR2 */
* Derived from linear interpolation: temp64 *= 1000; /* to get result in °mC */
* slope = 0.4297157 - (0.0015976 * 25C fuse)
* slope = (FACTOR2 - FACTOR1 * n1) / FACTOR0
* (Nmeas - n1) / (Tmeas - t1) = slope
* We want to reduce this down to the minimum computation necessary
* for each temperature read. Also, we want Tmeas in millicelsius
* and we don't want to lose precision from integer division. So...
* Tmeas = (Nmeas - n1) / slope + t1
* milli_Tmeas = 1000 * (Nmeas - n1) / slope + 1000 * t1
* milli_Tmeas = -1000 * (n1 - Nmeas) / slope + 1000 * t1
* Let constant c1 = (-1000 / slope)
* milli_Tmeas = (n1 - Nmeas) * c1 + 1000 * t1
* Let constant c2 = n1 *c1 + 1000 * t1
* milli_Tmeas = c2 - Nmeas * c1
*/
temp64 = FACTOR0;
temp64 *= 1000;
do_div(temp64, FACTOR1 * n1 - FACTOR2); do_div(temp64, FACTOR1 * n1 - FACTOR2);
data->c1 = temp64; data->c1 = temp64;
data->c2 = n1 * data->c1 + 1000 * t1; data->c2 = n1 * data->c1 + 1000 * t1;