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(lock: &data->lock);
154
155 /* voc = 500 / (1 + e^x) */
156 exp = sgp40_exp(exp: x, SGP40_CALC_POWER, rounds: 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(table: sgp40_crc8_table, pdata: (u8 *)&tg.rht_ticks, nbytes: 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(table: sgp40_crc8_table, pdata: (u8 *)&tg.temp_ticks, nbytes: 2, SGP40_CRC8_INIT);
186
187 mutex_unlock(lock: &data->lock);
188
189 ret = i2c_master_send(client, buf: (const char *)&tg, count: 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(msecs: 30);
195
196 ret = i2c_master_recv(client, buf: (u8 *)&tgres, count: 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(table: sgp40_crc8_table, pdata: (u8 *)&tgres.res_ticks, nbytes: 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: &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(lock: &data->lock);
237 return IIO_VAL_INT;
238 case IIO_HUMIDITYRELATIVE:
239 mutex_lock(&data->lock);
240 *val = data->rht;
241 mutex_unlock(lock: &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: &resistance_raw);
248 if (ret)
249 return ret;
250
251 ret = sgp40_calc_voc(data, resistance_raw, voc: &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(lock: &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(lock: &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(lock: &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(lock: &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(parent: dev, sizeof_priv: 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(table: 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");
384

source code of linux/drivers/iio/chemical/sgp40.c