iio:st_pressure: document sampling gains

Details scaling factors and offsets applied to raw temperature and pressure
samples.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/pressure/st_pressure_core.c

@@ -28,6 +28,72 @@
 #include <linux/iio/common/st_sensors.h>
 #include "st_pressure.h"
 
+/*
+ * About determining pressure scaling factors
+ * ------------------------------------------
+ *
+ * Datasheets specify typical pressure sensitivity so that pressure is computed
+ * according to the following equation :
+ *     pressure[mBar] = raw / sensitivity
+ * where :
+ *     raw          the 24 bits long raw sampled pressure
+ *     sensitivity  a scaling factor specified by the datasheet in LSB/mBar
+ *
+ * IIO ABI expects pressure to be expressed as kPascal, hence pressure should be
+ * computed according to :
+ *     pressure[kPascal] = pressure[mBar] / 10
+ *                       = raw / (sensitivity * 10)                          (1)
+ *
+ * Finally, st_press_read_raw() returns pressure scaling factor as an
+ * IIO_VAL_INT_PLUS_NANO with a zero integral part and "gain" as decimal part.
+ * Therefore, from (1), "gain" becomes :
+ *     gain = 10^9 / (sensitivity * 10)
+ *          = 10^8 / sensitivity
+ *
+ * About determining temperature scaling factors and offsets
+ * ---------------------------------------------------------
+ *
+ * Datasheets specify typical temperature sensitivity and offset so that
+ * temperature is computed according to the following equation :
+ *     temp[Celsius] = offset[Celsius] + (raw / sensitivity)
+ * where :
+ *     raw          the 16 bits long raw sampled temperature
+ *     offset       a constant specified by the datasheet in degree Celsius
+ *                  (sometimes zero)
+ *     sensitivity  a scaling factor specified by the datasheet in LSB/Celsius
+ *
+ * IIO ABI expects temperature to be expressed as milli degree Celsius such as
+ * user space should compute temperature according to :
+ *     temp[mCelsius] = temp[Celsius] * 10^3
+ *                    = (offset[Celsius] + (raw / sensitivity)) * 10^3
+ *                    = ((offset[Celsius] * sensitivity) + raw) *
+ *                      (10^3 / sensitivity)                                 (2)
+ *
+ * IIO ABI expects user space to apply offset and scaling factors to raw samples
+ * according to :
+ *     temp[mCelsius] = (OFFSET + raw) * SCALE
+ * where :
+ *     OFFSET an arbitrary constant exposed by device
+ *     SCALE  an arbitrary scaling factor exposed by device
+ *
+ * Matching OFFSET and SCALE with members of (2) gives :
+ *     OFFSET = offset[Celsius] * sensitivity                                (3)
+ *     SCALE  = 10^3 / sensitivity                                           (4)
+ *
+ * st_press_read_raw() returns temperature scaling factor as an
+ * IIO_VAL_FRACTIONAL with a 10^3 numerator and "gain2" as denominator.
+ * Therefore, from (3), "gain2" becomes :
+ *     gain2 = sensitivity
+ *
+ * When declared within channel, i.e. for a non zero specified offset,
+ * st_press_read_raw() will return the latter as an IIO_VAL_FRACTIONAL such as :
+ *     numerator = OFFSET * 10^3
+ *     denominator = 10^3
+ * giving from (4):
+ *     numerator = offset[Celsius] * 10^3 * sensitivity
+ *               = offset[mCelsius] * gain2
+ */
+
 #define MCELSIUS_PER_CELSIUS			1000
 
 /* Default pressure sensitivity */
@@ -48,7 +114,11 @@
 #define ST_PRESS_1_OUT_XL_ADDR			0x28
 #define ST_TEMP_1_OUT_L_ADDR			0x2b
 
-/* CUSTOM VALUES FOR LPS331AP SENSOR */
+/*
+ * CUSTOM VALUES FOR LPS331AP SENSOR
+ * See LPS331AP datasheet:
+ * http://www2.st.com/resource/en/datasheet/lps331ap.pdf
+ */
 #define ST_PRESS_LPS331AP_WAI_EXP		0xbb
 #define ST_PRESS_LPS331AP_ODR_ADDR		0x20
 #define ST_PRESS_LPS331AP_ODR_MASK		0x70
@@ -71,7 +141,9 @@
 #define ST_PRESS_LPS331AP_OD_IRQ_MASK		0x40
 #define ST_PRESS_LPS331AP_MULTIREAD_BIT		true
 
-/* CUSTOM VALUES FOR LPS001WP SENSOR */
+/*
+ * CUSTOM VALUES FOR THE OBSOLETE LPS001WP SENSOR
+ */
 
 /* LPS001WP pressure resolution */
 #define ST_PRESS_LPS001WP_LSB_PER_MBAR		16UL
@@ -94,7 +166,11 @@
 #define ST_PRESS_LPS001WP_OUT_L_ADDR		0x28
 #define ST_TEMP_LPS001WP_OUT_L_ADDR		0x2a
 
-/* CUSTOM VALUES FOR LPS25H SENSOR */
+/*
+ * CUSTOM VALUES FOR LPS25H SENSOR
+ * See LPS25H datasheet:
+ * http://www2.st.com/resource/en/datasheet/lps25h.pdf
+ */
 #define ST_PRESS_LPS25H_WAI_EXP			0xbd
 #define ST_PRESS_LPS25H_ODR_ADDR		0x20
 #define ST_PRESS_LPS25H_ODR_MASK		0x70
@@ -117,7 +193,11 @@
 #define ST_PRESS_LPS25H_OUT_XL_ADDR		0x28
 #define ST_TEMP_LPS25H_OUT_L_ADDR		0x2b
 
-/* CUSTOM VALUES FOR LPS22HB SENSOR */
+/*
+ * CUSTOM VALUES FOR LPS22HB SENSOR
+ * See LPS22HB datasheet:
+ * http://www2.st.com/resource/en/datasheet/lps22hb.pdf
+ */
 #define ST_PRESS_LPS22HB_WAI_EXP		0xb1
 #define ST_PRESS_LPS22HB_ODR_ADDR		0x10
 #define ST_PRESS_LPS22HB_ODR_MASK		0x70
@@ -413,6 +493,10 @@ static const struct st_sensor_settings st_press_sensors_settings[] = {
 		},
 		.fs = {
 			.fs_avl = {
+				/*
+				 * Sensitivity values as defined in table 3 of
+				 * LPS22HB datasheet.
+				 */
 				[0] = {
 					.num = ST_PRESS_FS_AVL_1260MB,
 					.gain = ST_PRESS_KPASCAL_NANO_SCALE,