iio: light: Add gain-time-scale helpers

Some light sensors can adjust both the HW-gain and integration time.
There are cases where adjusting the integration time has similar impact
to the scale of the reported values as gain setting has.

IIO users do typically expect to handle scale by a single writable 'scale'
entry. Driver should then adjust the gain/time accordingly.

It however is difficult for a driver to know whether it should change
gain or integration time to meet the requested scale. Usually it is
preferred to have longer integration time which usually improves
accuracy, but there may be use-cases where long measurement times can be
an issue. Thus it can be preferable to allow also changing the
integration time - but mitigate the scale impact by also changing the gain
underneath. Eg, if integration time change doubles the measured values,
the driver can reduce the HW-gain to half.

The theory of the computations of gain-time-scale is simple. However,
some people (undersigned) got that implemented wrong for more than once.

Add some gain-time-scale helpers in order to not dublicate errors in all
drivers needing these computations.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/268d418e7cffcdaa2ece6738478bbc57692c213e.1680263956.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/Kconfig

@@ -30,6 +30,9 @@ config IIO_CONFIGFS
 	  (e.g. software triggers). For more info see
 	  Documentation/iio/iio_configfs.rst.
 
+config IIO_GTS_HELPER
+	tristate
+
 config IIO_TRIGGER
 	bool "Enable triggered sampling support"
 	help

drivers/iio/Makefile

@@ -9,6 +9,7 @@ industrialio-$(CONFIG_IIO_BUFFER) += industrialio-buffer.o
 industrialio-$(CONFIG_IIO_TRIGGER) += industrialio-trigger.o
 
 obj-$(CONFIG_IIO_CONFIGFS) += industrialio-configfs.o
+obj-$(CONFIG_IIO_GTS_HELPER) += industrialio-gts-helper.o
 obj-$(CONFIG_IIO_SW_DEVICE) += industrialio-sw-device.o
 obj-$(CONFIG_IIO_SW_TRIGGER) += industrialio-sw-trigger.o
 obj-$(CONFIG_IIO_TRIGGERED_EVENT) += industrialio-triggered-event.o

include/linux/iio/iio-gts-helper.h

@@ -0,0 +1,206 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/* gain-time-scale conversion helpers for IIO light sensors
+ *
+ * Copyright (c) 2023 Matti Vaittinen <mazziesaccount@gmail.com>
+ */
+
+#ifndef __IIO_GTS_HELPER__
+#define __IIO_GTS_HELPER__
+
+#include <linux/types.h>
+
+struct device;
+
+/**
+ * struct iio_gain_sel_pair - gain - selector values
+ *
+ * In many cases devices like light sensors allow setting signal amplification
+ * (gain) using a register interface. This structure describes amplification
+ * and corresponding selector (register value)
+ *
+ * @gain:	Gain (multiplication) value. Gain must be positive, negative
+ *		values are reserved for error handling.
+ * @sel:	Selector (usually register value) used to indicate this gain.
+ *		NOTE: Only selectors >= 0 supported.
+ */
+struct iio_gain_sel_pair {
+	int gain;
+	int sel;
+};
+
+/**
+ * struct iio_itime_sel_mul - integration time description
+ *
+ * In many cases devices like light sensors allow setting the duration of
+ * collecting data. Typically this duration has also an impact to the magnitude
+ * of measured values (gain). This structure describes the relation of
+ * integration time and amplification as well as corresponding selector
+ * (register value).
+ *
+ * An example could be a sensor allowing 50, 100, 200 and 400 mS times. The
+ * respective multiplication values could be 50 mS => 1, 100 mS => 2,
+ * 200 mS => 4 and 400 mS => 8 assuming the impact of integration time would be
+ * linear in a way that when collecting data for 50 mS caused value X, doubling
+ * the data collection time caused value 2X etc.
+ *
+ * @time_us:	Integration time in microseconds. Time values must be positive,
+ *		negative values are reserved for error handling.
+ * @sel:	Selector (usually register value) used to indicate this time
+ *		NOTE: Only selectors >= 0 supported.
+ * @mul:	Multiplication to the values caused by this time.
+ *		NOTE: Only multipliers > 0 supported.
+ */
+struct iio_itime_sel_mul {
+	int time_us;
+	int sel;
+	int mul;
+};
+
+struct iio_gts {
+	u64 max_scale;
+	const struct iio_gain_sel_pair *hwgain_table;
+	int num_hwgain;
+	const struct iio_itime_sel_mul *itime_table;
+	int num_itime;
+	int **per_time_avail_scale_tables;
+	int *avail_all_scales_table;
+	int num_avail_all_scales;
+	int *avail_time_tables;
+	int num_avail_time_tables;
+};
+
+#define GAIN_SCALE_GAIN(_gain, _sel)			\
+{							\
+	.gain = (_gain),				\
+	.sel = (_sel),					\
+}
+
+#define GAIN_SCALE_ITIME_US(_itime, _sel, _mul)		\
+{							\
+	.time_us = (_itime),				\
+	.sel = (_sel),					\
+	.mul = (_mul),					\
+}
+
+static inline const struct iio_itime_sel_mul *
+iio_gts_find_itime_by_time(struct iio_gts *gts, int time)
+{
+	int i;
+
+	if (!gts->num_itime)
+		return NULL;
+
+	for (i = 0; i < gts->num_itime; i++)
+		if (gts->itime_table[i].time_us == time)
+			return &gts->itime_table[i];
+
+	return NULL;
+}
+
+static inline const struct iio_itime_sel_mul *
+iio_gts_find_itime_by_sel(struct iio_gts *gts, int sel)
+{
+	int i;
+
+	for (i = 0; i < gts->num_itime; i++)
+		if (gts->itime_table[i].sel == sel)
+			return &gts->itime_table[i];
+
+	return NULL;
+}
+
+int devm_iio_init_iio_gts(struct device *dev, int max_scale_int, int max_scale_nano,
+			  const struct iio_gain_sel_pair *gain_tbl, int num_gain,
+			  const struct iio_itime_sel_mul *tim_tbl, int num_times,
+			  struct iio_gts *gts);
+/**
+ * iio_gts_find_int_time_by_sel - find integration time matching a selector
+ * @gts:	Gain time scale descriptor
+ * @sel:	selector for which matching integration time is searched for
+ *
+ * Return:	integration time matching given selector or -EINVAL if
+ *		integration time was not found.
+ */
+static inline int iio_gts_find_int_time_by_sel(struct iio_gts *gts, int sel)
+{
+	const struct iio_itime_sel_mul *itime;
+
+	itime = iio_gts_find_itime_by_sel(gts, sel);
+	if (!itime)
+		return -EINVAL;
+
+	return itime->time_us;
+}
+
+/**
+ * iio_gts_find_sel_by_int_time - find selector matching integration time
+ * @gts:	Gain time scale descriptor
+ * @gain:	HW-gain for which matching selector is searched for
+ *
+ * Return:	a selector matching given integration time or -EINVAL if
+ *		selector was not found.
+ */
+static inline int iio_gts_find_sel_by_int_time(struct iio_gts *gts, int time)
+{
+	const struct iio_itime_sel_mul *itime;
+
+	itime = iio_gts_find_itime_by_time(gts, time);
+	if (!itime)
+		return -EINVAL;
+
+	return itime->sel;
+}
+
+/**
+ * iio_gts_valid_time - check if given integration time is valid
+ * @gts:	Gain time scale descriptor
+ * @time_us:	Integration time to check
+ *
+ * Return:	True if given time is supported by device. False if not.
+ */
+static inline bool iio_gts_valid_time(struct iio_gts *gts, int time_us)
+{
+	return iio_gts_find_itime_by_time(gts, time_us) != NULL;
+}
+
+int iio_gts_find_sel_by_gain(struct iio_gts *gts, int gain);
+
+/**
+ * iio_gts_valid_gain - check if given HW-gain is valid
+ * @gts:	Gain time scale descriptor
+ * @gain:	HW-gain to check
+ *
+ * Return:	True if given time is supported by device. False if not.
+ */
+static inline bool iio_gts_valid_gain(struct iio_gts *gts, int gain)
+{
+	return iio_gts_find_sel_by_gain(gts, gain) >= 0;
+}
+
+int iio_find_closest_gain_low(struct iio_gts *gts, int gain, bool *in_range);
+int iio_gts_find_gain_by_sel(struct iio_gts *gts, int sel);
+int iio_gts_get_min_gain(struct iio_gts *gts);
+int iio_gts_find_int_time_by_sel(struct iio_gts *gts, int sel);
+int iio_gts_find_sel_by_int_time(struct iio_gts *gts, int time);
+
+int iio_gts_total_gain_to_scale(struct iio_gts *gts, int total_gain,
+				int *scale_int, int *scale_nano);
+int iio_gts_find_gain_sel_for_scale_using_time(struct iio_gts *gts, int time_sel,
+					       int scale_int, int scale_nano,
+					       int *gain_sel);
+int iio_gts_get_scale(struct iio_gts *gts, int gain, int time, int *scale_int,
+		      int *scale_nano);
+int iio_gts_find_new_gain_sel_by_old_gain_time(struct iio_gts *gts,
+					       int old_gain, int old_time_sel,
+					       int new_time_sel, int *new_gain);
+int iio_gts_find_new_gain_by_old_gain_time(struct iio_gts *gts, int old_gain,
+					   int old_time, int new_time,
+					   int *new_gain);
+int iio_gts_avail_times(struct iio_gts *gts,  const int **vals, int *type,
+			int *length);
+int iio_gts_all_avail_scales(struct iio_gts *gts, const int **vals, int *type,
+			     int *length);
+int iio_gts_avail_scales_for_time(struct iio_gts *gts, int time,
+				  const int **vals, int *type, int *length);
+
+#endif