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Commit c5bbdb4b authored by Uwe Kleine-König's avatar Uwe Kleine-König Committed by Eduardo Valentin
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thermal: imx: improve comments describing algorithm for temp calculation 



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: default avatarLeonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: default avatarUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: default avatarEduardo Valentin <edubezval@gmail.com>
parent e4bb2240

drivers/thermal/imx_thermal.c

+19 −23
Original line number Diff line number Diff line
@@ -359,32 +359,28 @@ static int imx_init_calib(struct platform_device *pdev, u32 ocotp_ana1) 
	}



	/*

	 * Sensor data layout:

	 *   [31:20] - sensor value @ 25C

	 * Use universal formula now and only need sensor value @ 25C

	 * slope = 0.4297157 - (0.0015976 * 25C fuse)

	 * The sensor is calibrated at 25 °C (aka T1) and the value measured

	 * (aka N1) at this temperature is provided in bits [31:20] in the

	 * i.MX's OCOTP value ANA1.

	 * 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;

	t1 = 25; /* t1 always 25C */

	t1 = 25; /* °C */



	/*

	 * Derived from linear interpolation:

	 * 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;

	temp64 = FACTOR0; /* 10^7 for FACTOR1 and FACTOR2 */

	temp64 *= 1000; /* to get result in °mC */

	do_div(temp64, FACTOR1 * n1 - FACTOR2);

	data->c1 = temp64;

	data->c2 = n1 * data->c1 + 1000 * t1;