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