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