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- // SPDX-License-Identifier: GPL-2.0+
- /*
- * sgp40.c - Support for Sensirion SGP40 Gas Sensor
- *
- * Copyright (C) 2021 Andreas Klinger <[email protected]>
- *
- * I2C slave address: 0x59
- *
- * Datasheet can be found here:
- * https://www.sensirion.com/file/datasheet_sgp40
- *
- * There are two functionalities supported:
- *
- * 1) read raw logarithmic resistance value from sensor
- * --> useful to pass it to the algorithm of the sensor vendor for
- * measuring deteriorations and improvements of air quality.
- *
- * 2) calculate an estimated absolute voc index (0 - 500 index points) for
- * measuring the air quality.
- * For this purpose the value of the resistance for which the voc index
- * will be 250 can be set up using calibbias.
- *
- * Compensation values of relative humidity and temperature can be set up
- * by writing to the out values of temp and humidityrelative.
- */
- #include <linux/delay.h>
- #include <linux/crc8.h>
- #include <linux/module.h>
- #include <linux/mutex.h>
- #include <linux/i2c.h>
- #include <linux/iio/iio.h>
- /*
- * floating point calculation of voc is done as integer
- * where numbers are multiplied by 1 << SGP40_CALC_POWER
- */
- #define SGP40_CALC_POWER 14
- #define SGP40_CRC8_POLYNOMIAL 0x31
- #define SGP40_CRC8_INIT 0xff
- DECLARE_CRC8_TABLE(sgp40_crc8_table);
- struct sgp40_data {
- struct device *dev;
- struct i2c_client *client;
- int rht;
- int temp;
- int res_calibbias;
- /* Prevent concurrent access to rht, tmp, calibbias */
- struct mutex lock;
- };
- struct sgp40_tg_measure {
- u8 command[2];
- __be16 rht_ticks;
- u8 rht_crc;
- __be16 temp_ticks;
- u8 temp_crc;
- } __packed;
- struct sgp40_tg_result {
- __be16 res_ticks;
- u8 res_crc;
- } __packed;
- static const struct iio_chan_spec sgp40_channels[] = {
- {
- .type = IIO_CONCENTRATION,
- .channel2 = IIO_MOD_VOC,
- .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
- },
- {
- .type = IIO_RESISTANCE,
- .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
- BIT(IIO_CHAN_INFO_CALIBBIAS),
- },
- {
- .type = IIO_TEMP,
- .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
- .output = 1,
- },
- {
- .type = IIO_HUMIDITYRELATIVE,
- .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
- .output = 1,
- },
- };
- /*
- * taylor approximation of e^x:
- * y = 1 + x + x^2 / 2 + x^3 / 6 + x^4 / 24 + ... + x^n / n!
- *
- * Because we are calculating x real value multiplied by 2^power we get
- * an additional 2^power^n to divide for every element. For a reasonable
- * precision this would overflow after a few iterations. Therefore we
- * divide the x^n part whenever its about to overflow (xmax).
- */
- static u32 sgp40_exp(int exp, u32 power, u32 rounds)
- {
- u32 x, y, xp;
- u32 factorial, divider, xmax;
- int sign = 1;
- int i;
- if (exp == 0)
- return 1 << power;
- else if (exp < 0) {
- sign = -1;
- exp *= -1;
- }
- xmax = 0x7FFFFFFF / exp;
- x = exp;
- xp = 1;
- factorial = 1;
- y = 1 << power;
- divider = 0;
- for (i = 1; i <= rounds; i++) {
- xp *= x;
- factorial *= i;
- y += (xp >> divider) / factorial;
- divider += power;
- /* divide when next multiplication would overflow */
- if (xp >= xmax) {
- xp >>= power;
- divider -= power;
- }
- }
- if (sign == -1)
- return (1 << (power * 2)) / y;
- else
- return y;
- }
- static int sgp40_calc_voc(struct sgp40_data *data, u16 resistance_raw, int *voc)
- {
- int x;
- u32 exp = 0;
- /* we calculate as a multiple of 16384 (2^14) */
- mutex_lock(&data->lock);
- x = ((int)resistance_raw - data->res_calibbias) * 106;
- mutex_unlock(&data->lock);
- /* voc = 500 / (1 + e^x) */
- exp = sgp40_exp(x, SGP40_CALC_POWER, 18);
- *voc = 500 * ((1 << (SGP40_CALC_POWER * 2)) / ((1<<SGP40_CALC_POWER) + exp));
- dev_dbg(data->dev, "raw: %d res_calibbias: %d x: %d exp: %d voc: %d\n",
- resistance_raw, data->res_calibbias, x, exp, *voc);
- return 0;
- }
- static int sgp40_measure_resistance_raw(struct sgp40_data *data, u16 *resistance_raw)
- {
- int ret;
- struct i2c_client *client = data->client;
- u32 ticks;
- u16 ticks16;
- u8 crc;
- struct sgp40_tg_measure tg = {.command = {0x26, 0x0F}};
- struct sgp40_tg_result tgres;
- mutex_lock(&data->lock);
- ticks = (data->rht / 10) * 65535 / 10000;
- ticks16 = (u16)clamp(ticks, 0u, 65535u); /* clamp between 0 .. 100 %rH */
- tg.rht_ticks = cpu_to_be16(ticks16);
- tg.rht_crc = crc8(sgp40_crc8_table, (u8 *)&tg.rht_ticks, 2, SGP40_CRC8_INIT);
- ticks = ((data->temp + 45000) / 10 ) * 65535 / 17500;
- ticks16 = (u16)clamp(ticks, 0u, 65535u); /* clamp between -45 .. +130 °C */
- tg.temp_ticks = cpu_to_be16(ticks16);
- tg.temp_crc = crc8(sgp40_crc8_table, (u8 *)&tg.temp_ticks, 2, SGP40_CRC8_INIT);
- mutex_unlock(&data->lock);
- ret = i2c_master_send(client, (const char *)&tg, sizeof(tg));
- if (ret != sizeof(tg)) {
- dev_warn(data->dev, "i2c_master_send ret: %d sizeof: %zu\n", ret, sizeof(tg));
- return -EIO;
- }
- msleep(30);
- ret = i2c_master_recv(client, (u8 *)&tgres, sizeof(tgres));
- if (ret < 0)
- return ret;
- if (ret != sizeof(tgres)) {
- dev_warn(data->dev, "i2c_master_recv ret: %d sizeof: %zu\n", ret, sizeof(tgres));
- return -EIO;
- }
- crc = crc8(sgp40_crc8_table, (u8 *)&tgres.res_ticks, 2, SGP40_CRC8_INIT);
- if (crc != tgres.res_crc) {
- dev_err(data->dev, "CRC error while measure-raw\n");
- return -EIO;
- }
- *resistance_raw = be16_to_cpu(tgres.res_ticks);
- return 0;
- }
- static int sgp40_read_raw(struct iio_dev *indio_dev,
- struct iio_chan_spec const *chan, int *val,
- int *val2, long mask)
- {
- struct sgp40_data *data = iio_priv(indio_dev);
- int ret, voc;
- u16 resistance_raw;
- switch (mask) {
- case IIO_CHAN_INFO_RAW:
- switch (chan->type) {
- case IIO_RESISTANCE:
- ret = sgp40_measure_resistance_raw(data, &resistance_raw);
- if (ret)
- return ret;
- *val = resistance_raw;
- return IIO_VAL_INT;
- case IIO_TEMP:
- mutex_lock(&data->lock);
- *val = data->temp;
- mutex_unlock(&data->lock);
- return IIO_VAL_INT;
- case IIO_HUMIDITYRELATIVE:
- mutex_lock(&data->lock);
- *val = data->rht;
- mutex_unlock(&data->lock);
- return IIO_VAL_INT;
- default:
- return -EINVAL;
- }
- case IIO_CHAN_INFO_PROCESSED:
- ret = sgp40_measure_resistance_raw(data, &resistance_raw);
- if (ret)
- return ret;
- ret = sgp40_calc_voc(data, resistance_raw, &voc);
- if (ret)
- return ret;
- *val = voc / (1 << SGP40_CALC_POWER);
- /*
- * calculation should fit into integer, where:
- * voc <= (500 * 2^SGP40_CALC_POWER) = 8192000
- * (with SGP40_CALC_POWER = 14)
- */
- *val2 = ((voc % (1 << SGP40_CALC_POWER)) * 244) / (1 << (SGP40_CALC_POWER - 12));
- dev_dbg(data->dev, "voc: %d val: %d.%06d\n", voc, *val, *val2);
- return IIO_VAL_INT_PLUS_MICRO;
- case IIO_CHAN_INFO_CALIBBIAS:
- mutex_lock(&data->lock);
- *val = data->res_calibbias;
- mutex_unlock(&data->lock);
- return IIO_VAL_INT;
- default:
- return -EINVAL;
- }
- }
- static int sgp40_write_raw(struct iio_dev *indio_dev,
- struct iio_chan_spec const *chan, int val,
- int val2, long mask)
- {
- struct sgp40_data *data = iio_priv(indio_dev);
- switch (mask) {
- case IIO_CHAN_INFO_RAW:
- switch (chan->type) {
- case IIO_TEMP:
- if ((val < -45000) || (val > 130000))
- return -EINVAL;
- mutex_lock(&data->lock);
- data->temp = val;
- mutex_unlock(&data->lock);
- return 0;
- case IIO_HUMIDITYRELATIVE:
- if ((val < 0) || (val > 100000))
- return -EINVAL;
- mutex_lock(&data->lock);
- data->rht = val;
- mutex_unlock(&data->lock);
- return 0;
- default:
- return -EINVAL;
- }
- case IIO_CHAN_INFO_CALIBBIAS:
- if ((val < 20000) || (val > 52768))
- return -EINVAL;
- mutex_lock(&data->lock);
- data->res_calibbias = val;
- mutex_unlock(&data->lock);
- return 0;
- }
- return -EINVAL;
- }
- static const struct iio_info sgp40_info = {
- .read_raw = sgp40_read_raw,
- .write_raw = sgp40_write_raw,
- };
- static int sgp40_probe(struct i2c_client *client,
- const struct i2c_device_id *id)
- {
- struct device *dev = &client->dev;
- struct iio_dev *indio_dev;
- struct sgp40_data *data;
- int ret;
- indio_dev = devm_iio_device_alloc(dev, sizeof(*data));
- if (!indio_dev)
- return -ENOMEM;
- data = iio_priv(indio_dev);
- data->client = client;
- data->dev = dev;
- crc8_populate_msb(sgp40_crc8_table, SGP40_CRC8_POLYNOMIAL);
- mutex_init(&data->lock);
- /* set default values */
- data->rht = 50000; /* 50 % */
- data->temp = 25000; /* 25 °C */
- data->res_calibbias = 30000; /* resistance raw value for voc index of 250 */
- indio_dev->info = &sgp40_info;
- indio_dev->name = id->name;
- indio_dev->modes = INDIO_DIRECT_MODE;
- indio_dev->channels = sgp40_channels;
- indio_dev->num_channels = ARRAY_SIZE(sgp40_channels);
- ret = devm_iio_device_register(dev, indio_dev);
- if (ret)
- dev_err(dev, "failed to register iio device\n");
- return ret;
- }
- static const struct i2c_device_id sgp40_id[] = {
- { "sgp40" },
- { }
- };
- MODULE_DEVICE_TABLE(i2c, sgp40_id);
- static const struct of_device_id sgp40_dt_ids[] = {
- { .compatible = "sensirion,sgp40" },
- { }
- };
- MODULE_DEVICE_TABLE(of, sgp40_dt_ids);
- static struct i2c_driver sgp40_driver = {
- .driver = {
- .name = "sgp40",
- .of_match_table = sgp40_dt_ids,
- },
- .probe = sgp40_probe,
- .id_table = sgp40_id,
- };
- module_i2c_driver(sgp40_driver);
- MODULE_AUTHOR("Andreas Klinger <[email protected]>");
- MODULE_DESCRIPTION("Sensirion SGP40 gas sensor");
- MODULE_LICENSE("GPL v2");
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