xref: /openbmc/linux/drivers/iio/chemical/sgp40.c (revision 7cf15f42)
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 
43 DECLARE_CRC8_TABLE(sgp40_crc8_table);
44 
45 struct 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 
55 struct 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 
63 struct sgp40_tg_result {
64 	__be16	res_ticks;
65 	u8	res_crc;
66 } __packed;
67 
68 static 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 
sgp40_exp(int exp,u32 power,u32 rounds)101 static 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 
sgp40_calc_voc(struct sgp40_data * data,u16 resistance_raw,int * voc)140 static 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 
sgp40_measure_resistance_raw(struct sgp40_data * data,u16 * resistance_raw)160 static 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 
sgp40_read_raw(struct iio_dev * indio_dev,struct iio_chan_spec const * chan,int * val,int * val2,long mask)210 static 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 
sgp40_write_raw(struct iio_dev * indio_dev,struct iio_chan_spec const * chan,int val,int val2,long mask)269 static 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 
309 static const struct iio_info sgp40_info = {
310 	.read_raw	= sgp40_read_raw,
311 	.write_raw	= sgp40_write_raw,
312 };
313 
sgp40_probe(struct i2c_client * client)314 static 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 
352 static const struct i2c_device_id sgp40_id[] = {
353 	{ "sgp40" },
354 	{ }
355 };
356 
357 MODULE_DEVICE_TABLE(i2c, sgp40_id);
358 
359 static const struct of_device_id sgp40_dt_ids[] = {
360 	{ .compatible = "sensirion,sgp40" },
361 	{ }
362 };
363 
364 MODULE_DEVICE_TABLE(of, sgp40_dt_ids);
365 
366 static 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 };
374 module_i2c_driver(sgp40_driver);
375 
376 MODULE_AUTHOR("Andreas Klinger <ak@it-klinger.de>");
377 MODULE_DESCRIPTION("Sensirion SGP40 gas sensor");
378 MODULE_LICENSE("GPL v2");
379