sgp40.c

  1// SPDX-License-Identifier: GPL-2.0+
  2/*
  3 * sgp40.c - Support for Sensirion SGP40 Gas Sensor
  4 *
  5 * Copyright (C) 2021 Andreas Klinger <ak@it-klinger.de>
  6 *
  7 * I2C slave address: 0x59
  8 *
  9 * Datasheet can be found here:
 10 * https://www.sensirion.com/file/datasheet_sgp40
 11 *
 12 * There are two functionalities supported:
 13 *
 14 * 1) read raw logarithmic resistance value from sensor
 15 *    --> useful to pass it to the algorithm of the sensor vendor for
 16 *    measuring deteriorations and improvements of air quality.
 17 *    It can be read from the attribute in_resistance_raw.
 18 *
 19 * 2) calculate an estimated absolute voc index (in_concentration_input)
 20 *    with 0 - 500 index points) for measuring the air quality.
 21 *    For this purpose the value of the resistance for which the voc index
 22 *    will be 250 can be set up using in_resistance_calibbias (default 30000).
 23 *
 24 *    The voc index is calculated as:
 25 *      x = (in_resistance_raw - in_resistance_calibbias) * 0.65
 26 *      in_concentration_input = 500 / (1 + e^x)
 27 *
 28 * Compensation values of relative humidity and temperature can be set up
 29 * by writing to the out values of temp and humidityrelative.
 30 */
 31
 32#include <linux/delay.h>
 33#include <linux/crc8.h>
 34#include <linux/module.h>
 35#include <linux/mutex.h>
 36#include <linux/i2c.h>
 37#include <linux/iio/iio.h>
 38
 39/*
 40 * floating point calculation of voc is done as integer
 41 * where numbers are multiplied by 1 << SGP40_CALC_POWER
 42 */
 43#define SGP40_CALC_POWER	14
 44
 45#define SGP40_CRC8_POLYNOMIAL	0x31
 46#define SGP40_CRC8_INIT		0xff
 47
 48DECLARE_CRC8_TABLE(sgp40_crc8_table);
 49
 50struct sgp40_data {
 51	struct device		*dev;
 52	struct i2c_client	*client;
 53	int			rht;
 54	int			temp;
 55	int			res_calibbias;
 56	/* Prevent concurrent access to rht, tmp, calibbias */
 57	struct mutex		lock;
 58};
 59
 60struct sgp40_tg_measure {
 61	u8	command[2];
 62	__be16	rht_ticks;
 63	u8	rht_crc;
 64	__be16	temp_ticks;
 65	u8	temp_crc;
 66} __packed;
 67
 68struct sgp40_tg_result {
 69	__be16	res_ticks;
 70	u8	res_crc;
 71} __packed;
 72
 73static const struct iio_chan_spec sgp40_channels[] = {
 74	{
 75		.type = IIO_CONCENTRATION,
 76		.channel2 = IIO_MOD_VOC,
 77		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
 78	},
 79	{
 80		.type = IIO_RESISTANCE,
 81		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
 82			BIT(IIO_CHAN_INFO_CALIBBIAS),
 83	},
 84	{
 85		.type = IIO_TEMP,
 86		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
 87		.output = 1,
 88	},
 89	{
 90		.type = IIO_HUMIDITYRELATIVE,
 91		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
 92		.output = 1,
 93	},
 94};
 95
 96/*
 97 * taylor approximation of e^x:
 98 * y = 1 + x + x^2 / 2 + x^3 / 6 + x^4 / 24 + ... + x^n / n!
 99 *
100 * Because we are calculating x real value multiplied by 2^power we get
101 * an additional 2^power^n to divide for every element. For a reasonable
102 * precision this would overflow after a few iterations. Therefore we
103 * divide the x^n part whenever its about to overflow (xmax).
104 */
105
106static u32 sgp40_exp(int exp, u32 power, u32 rounds)
107{
108        u32 x, y, xp;
109        u32 factorial, divider, xmax;
110        int sign = 1;
111	int i;
112
113        if (exp == 0)
114                return 1 << power;
115        else if (exp < 0) {
116                sign = -1;
117                exp *= -1;
118        }
119
120        xmax = 0x7FFFFFFF / exp;
121        x = exp;
122        xp = 1;
123        factorial = 1;
124        y = 1 << power;
125        divider = 0;
126
127        for (i = 1; i <= rounds; i++) {
128                xp *= x;
129                factorial *= i;
130                y += (xp >> divider) / factorial;
131                divider += power;
132                /* divide when next multiplication would overflow */
133                if (xp >= xmax) {
134                        xp >>= power;
135                        divider -= power;
136                }
137        }
138
139        if (sign == -1)
140                return (1 << (power * 2)) / y;
141        else
142                return y;
143}
144
145static int sgp40_calc_voc(struct sgp40_data *data, u16 resistance_raw, int *voc)
146{
147	int x;
148	u32 exp = 0;
149
150	/* we calculate as a multiple of 16384 (2^14) */
151	mutex_lock(&data->lock);
152	x = ((int)resistance_raw - data->res_calibbias) * 106;
153	mutex_unlock(&data->lock);
154
155	/* voc = 500 / (1 + e^x) */
156	exp = sgp40_exp(x, SGP40_CALC_POWER, 18);
157	*voc = 500 * ((1 << (SGP40_CALC_POWER * 2)) / ((1<<SGP40_CALC_POWER) + exp));
158
159	dev_dbg(data->dev, "raw: %d res_calibbias: %d x: %d exp: %d voc: %d\n",
160				resistance_raw, data->res_calibbias, x, exp, *voc);
161
162	return 0;
163}
164
165static int sgp40_measure_resistance_raw(struct sgp40_data *data, u16 *resistance_raw)
166{
167	int ret;
168	struct i2c_client *client = data->client;
169	u32 ticks;
170	u16 ticks16;
171	u8 crc;
172	struct sgp40_tg_measure tg = {.command = {0x26, 0x0F}};
173	struct sgp40_tg_result tgres;
174
175	mutex_lock(&data->lock);
176
177	ticks = (data->rht / 10) * 65535 / 10000;
178	ticks16 = (u16)clamp(ticks, 0u, 65535u); /* clamp between 0 .. 100 %rH */
179	tg.rht_ticks = cpu_to_be16(ticks16);
180	tg.rht_crc = crc8(sgp40_crc8_table, (u8 *)&tg.rht_ticks, 2, SGP40_CRC8_INIT);
181
182	ticks = ((data->temp + 45000) / 10 ) * 65535 / 17500;
183	ticks16 = (u16)clamp(ticks, 0u, 65535u); /* clamp between -45 .. +130 °C */
184	tg.temp_ticks = cpu_to_be16(ticks16);
185	tg.temp_crc = crc8(sgp40_crc8_table, (u8 *)&tg.temp_ticks, 2, SGP40_CRC8_INIT);
186
187	mutex_unlock(&data->lock);
188
189	ret = i2c_master_send(client, (const char *)&tg, sizeof(tg));
190	if (ret != sizeof(tg)) {
191		dev_warn(data->dev, "i2c_master_send ret: %d sizeof: %zu\n", ret, sizeof(tg));
192		return -EIO;
193	}
194	msleep(30);
195
196	ret = i2c_master_recv(client, (u8 *)&tgres, sizeof(tgres));
197	if (ret < 0)
198		return ret;
199	if (ret != sizeof(tgres)) {
200		dev_warn(data->dev, "i2c_master_recv ret: %d sizeof: %zu\n", ret, sizeof(tgres));
201		return -EIO;
202	}
203
204	crc = crc8(sgp40_crc8_table, (u8 *)&tgres.res_ticks, 2, SGP40_CRC8_INIT);
205	if (crc != tgres.res_crc) {
206		dev_err(data->dev, "CRC error while measure-raw\n");
207		return -EIO;
208	}
209
210	*resistance_raw = be16_to_cpu(tgres.res_ticks);
211
212	return 0;
213}
214
215static int sgp40_read_raw(struct iio_dev *indio_dev,
216			struct iio_chan_spec const *chan, int *val,
217			int *val2, long mask)
218{
219	struct sgp40_data *data = iio_priv(indio_dev);
220	int ret, voc;
221	u16 resistance_raw;
222
223	switch (mask) {
224	case IIO_CHAN_INFO_RAW:
225		switch (chan->type) {
226		case IIO_RESISTANCE:
227			ret = sgp40_measure_resistance_raw(data, &resistance_raw);
228			if (ret)
229				return ret;
230
231			*val = resistance_raw;
232			return IIO_VAL_INT;
233		case IIO_TEMP:
234			mutex_lock(&data->lock);
235			*val = data->temp;
236			mutex_unlock(&data->lock);
237			return IIO_VAL_INT;
238		case IIO_HUMIDITYRELATIVE:
239			mutex_lock(&data->lock);
240			*val = data->rht;
241			mutex_unlock(&data->lock);
242			return IIO_VAL_INT;
243		default:
244			return -EINVAL;
245		}
246	case IIO_CHAN_INFO_PROCESSED:
247		ret = sgp40_measure_resistance_raw(data, &resistance_raw);
248		if (ret)
249			return ret;
250
251		ret = sgp40_calc_voc(data, resistance_raw, &voc);
252		if (ret)
253			return ret;
254
255		*val = voc / (1 << SGP40_CALC_POWER);
256		/*
257		 * calculation should fit into integer, where:
258		 * voc <= (500 * 2^SGP40_CALC_POWER) = 8192000
259		 * (with SGP40_CALC_POWER = 14)
260		 */
261		*val2 = ((voc % (1 << SGP40_CALC_POWER)) * 244) / (1 << (SGP40_CALC_POWER - 12));
262		dev_dbg(data->dev, "voc: %d val: %d.%06d\n", voc, *val, *val2);
263		return IIO_VAL_INT_PLUS_MICRO;
264	case IIO_CHAN_INFO_CALIBBIAS:
265		mutex_lock(&data->lock);
266		*val = data->res_calibbias;
267		mutex_unlock(&data->lock);
268		return IIO_VAL_INT;
269	default:
270		return -EINVAL;
271	}
272}
273
274static int sgp40_write_raw(struct iio_dev *indio_dev,
275			struct iio_chan_spec const *chan, int val,
276			int val2, long mask)
277{
278	struct sgp40_data *data = iio_priv(indio_dev);
279
280	switch (mask) {
281	case IIO_CHAN_INFO_RAW:
282		switch (chan->type) {
283		case IIO_TEMP:
284			if ((val < -45000) || (val > 130000))
285				return -EINVAL;
286
287			mutex_lock(&data->lock);
288			data->temp = val;
289			mutex_unlock(&data->lock);
290			return 0;
291		case IIO_HUMIDITYRELATIVE:
292			if ((val < 0) || (val > 100000))
293				return -EINVAL;
294
295			mutex_lock(&data->lock);
296			data->rht = val;
297			mutex_unlock(&data->lock);
298			return 0;
299		default:
300			return -EINVAL;
301		}
302	case IIO_CHAN_INFO_CALIBBIAS:
303		if ((val < 20000) || (val > 52768))
304			return -EINVAL;
305
306		mutex_lock(&data->lock);
307		data->res_calibbias = val;
308		mutex_unlock(&data->lock);
309		return 0;
310	}
311	return -EINVAL;
312}
313
314static const struct iio_info sgp40_info = {
315	.read_raw	= sgp40_read_raw,
316	.write_raw	= sgp40_write_raw,
317};
318
319static int sgp40_probe(struct i2c_client *client)
320{
321	const struct i2c_device_id *id = i2c_client_get_device_id(client);
322	struct device *dev = &client->dev;
323	struct iio_dev *indio_dev;
324	struct sgp40_data *data;
325	int ret;
326
327	indio_dev = devm_iio_device_alloc(dev, sizeof(*data));
328	if (!indio_dev)
329		return -ENOMEM;
330
331	data = iio_priv(indio_dev);
332	data->client = client;
333	data->dev = dev;
334
335	crc8_populate_msb(sgp40_crc8_table, SGP40_CRC8_POLYNOMIAL);
336
337	mutex_init(&data->lock);
338
339	/* set default values */
340	data->rht = 50000;		/* 50 % */
341	data->temp = 25000;		/* 25 °C */
342	data->res_calibbias = 30000;	/* resistance raw value for voc index of 250 */
343
344	indio_dev->info = &sgp40_info;
345	indio_dev->name = id->name;
346	indio_dev->modes = INDIO_DIRECT_MODE;
347	indio_dev->channels = sgp40_channels;
348	indio_dev->num_channels = ARRAY_SIZE(sgp40_channels);
349
350	ret = devm_iio_device_register(dev, indio_dev);
351	if (ret)
352		dev_err(dev, "failed to register iio device\n");
353
354	return ret;
355}
356
357static const struct i2c_device_id sgp40_id[] = {
358	{ "sgp40" },
359	{ }
360};
361
362MODULE_DEVICE_TABLE(i2c, sgp40_id);
363
364static const struct of_device_id sgp40_dt_ids[] = {
365	{ .compatible = "sensirion,sgp40" },
366	{ }
367};
368
369MODULE_DEVICE_TABLE(of, sgp40_dt_ids);
370
371static struct i2c_driver sgp40_driver = {
372	.driver = {
373		.name = "sgp40",
374		.of_match_table = sgp40_dt_ids,
375	},
376	.probe = sgp40_probe,
377	.id_table = sgp40_id,
378};
379module_i2c_driver(sgp40_driver);
380
381MODULE_AUTHOR("Andreas Klinger <ak@it-klinger.de>");
382MODULE_DESCRIPTION("Sensirion SGP40 gas sensor");
383MODULE_LICENSE("GPL v2");