Re: [PATCH v3] hwmon: (sht4x): add heater support

[Guenter Roeck <linux@roeck-us.net>]

On 8/26/24 05:58, Antoni Pokusinski wrote:
> Add support for manipulating the internal heater of sht4x devices.
> Enabling the heater removes condensed water from the sensor surface
> which disturbs the relative humidity measurements.
> 
> The heater can operate at three heating levels (20, 110 or 200
> milliwatts). Also, two heating durations may be selected (0.1 or 1s).
> Once the heating time elapses the heater is automatically switched off.
> 
> Signed-off-by: Antoni Pokusinski <apokusinski01@gmail.com>
> ---
> Hello Guenter,
> 
> Thanks for the review.
> I'm not sure if this short patch description above is now ok or shall I
> be even more detailed.
> The problem that I was facing was that when I was using the sensor for
> relative humidity measurements the results were always ~100% even though
> the correct value should be ~60%. Enabling the heater in periodic pulses
> to remove the condensation fixed the issue and the measurements were
> correct after that.
> Thus, I'm not sure if I should include that detailed arguments in the
> patch description or leave it as it is right now.
> 
> Regards,
> Antoni Pokusinski
> 
> --
> Changes since v2:
> * heater_enable_store: remove unnecessary if
> * Documentation: remove incorrect info about turning off the heater
> * be more specific in the patch description
> 
> Changes since v1:
> * explain the use case of the new attributes set
> * heater_enable attr: make it write-only
> * heater_enable_store: define cmd as u8 instead of u8*
> * heater_enable_store: remove unreachable data path
> * heater_enable_store: remove unnecessary lock
> * heater_enable_store: call i2c_master_send only if status==true
> * define attributes as DEVICE_ATTR_* instead of SENSOR_DEVICE_ATTR_*
> ---
>   Documentation/hwmon/sht4x.rst |  10 +++
>   drivers/hwmon/sht4x.c         | 126 +++++++++++++++++++++++++++++++++-
>   2 files changed, 135 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/hwmon/sht4x.rst b/Documentation/hwmon/sht4x.rst
> index daf21e763425..0b17f89fa1ab 100644
> --- a/Documentation/hwmon/sht4x.rst
> +++ b/Documentation/hwmon/sht4x.rst
> @@ -42,4 +42,14 @@ humidity1_input Measured humidity in %H
>   update_interval The minimum interval for polling the sensor,
>                   in milliseconds. Writable. Must be at least
>                   2000.
> +heater_power	The requested heater power, in milliwatts.
> +		Available values: 20, 110, 200 (default: 200).
> +heater_time	The requested operating time of the heater,
> +		in milliseconds.
> +		Available values: 100, 1000 (default 1000).
> +heater_enable	Enable the heater with the selected power
> +		and for the selected time in order to remove
> +		condensed water from the sensor surface. Write-only.
> +
> +			- 1: turn on
>   =============== ============================================
> diff --git a/drivers/hwmon/sht4x.c b/drivers/hwmon/sht4x.c
> index b8916d2735b5..20b50f7feac0 100644
> --- a/drivers/hwmon/sht4x.c
> +++ b/drivers/hwmon/sht4x.c
> @@ -11,6 +11,7 @@
>   #include <linux/crc8.h>
>   #include <linux/delay.h>
>   #include <linux/hwmon.h>
> +#include <linux/hwmon-sysfs.h>
>   #include <linux/i2c.h>
>   #include <linux/jiffies.h>
>   #include <linux/module.h>
> @@ -31,6 +32,12 @@
>    */
>   #define SHT4X_CMD_MEASURE_HPM	0b11111101
>   #define SHT4X_CMD_RESET		0b10010100
> +#define SHT4X_CMD_HEATER_20_1	0b00011110
> +#define SHT4X_CMD_HEATER_20_01	0b00010101
> +#define SHT4X_CMD_HEATER_110_1	0b00101111
> +#define SHT4X_CMD_HEATER_110_01	0b00100100
> +#define SHT4X_CMD_HEATER_200_1	0b00111001
> +#define SHT4X_CMD_HEATER_200_01 0b00110010
>   
>   #define SHT4X_CMD_LEN		1
>   #define SHT4X_CRC8_LEN		1
> @@ -54,6 +61,8 @@ DECLARE_CRC8_TABLE(sht4x_crc8_table);
>    * @last_updated: the previous time that the SHT4X was polled
>    * @temperature: the latest temperature value received from the SHT4X
>    * @humidity: the latest humidity value received from the SHT4X
> + * @heater_power: the power at which the heater will be started
> + * @heater_time: the time for which the heater will remain turned on
>    */
>   struct sht4x_data {
>   	struct i2c_client	*client;
> @@ -63,6 +72,8 @@ struct sht4x_data {
>   	long			last_updated;	/* in jiffies */
>   	s32			temperature;
>   	s32			humidity;
> +	u32			heater_power;	/* in milli-watts */
> +	u32			heater_time;	/* in milli-seconds */
>   };
>   
>   /**
> @@ -215,6 +226,117 @@ static int sht4x_hwmon_write(struct device *dev, enum hwmon_sensor_types type,
>   	}
>   }
>   
> +static ssize_t heater_enable_store(struct device *dev,
> +				   struct device_attribute *attr,
> +				   const char *buf,
> +				   size_t count)
> +{
> +	struct sht4x_data *data = dev_get_drvdata(dev);
> +	bool status;
> +	ssize_t ret;
> +	u8 cmd;
> +
> +	ret = kstrtobool(buf, &status);
> +	if (ret)
> +		return ret;
> +
> +	if (status) {
> +		if (data->heater_power == 20) {
> +			if (data->heater_time == 100)
> +				cmd = SHT4X_CMD_HEATER_20_01;
> +			else /* data->heater_time == 1000 */
> +				cmd = SHT4X_CMD_HEATER_20_1;
> +		} else if (data->heater_power == 110) {
> +			if (data->heater_time == 100)
> +				cmd = SHT4X_CMD_HEATER_110_01;
> +			else /* data->heater_time == 1000 */
> +				cmd = SHT4X_CMD_HEATER_110_1;
> +		} else { /* data->heater_power == 200 */
> +			if (data->heater_time == 100)
> +				cmd = SHT4X_CMD_HEATER_200_01;
> +			else /* data->heater_time == 1000 */
> +				cmd = SHT4X_CMD_HEATER_200_1;
> +		}
> +
> +		ret = i2c_master_send(data->client, &cmd, SHT4X_CMD_LEN);
> +	}

This still accepts (and ignores) writing "0" or "false".

On top of that, unless I misunderstand the chip documentation, attempts
to read the humidity before the "turn on heater" command completes will
result in a NACK from the chip. This is not currently handled by the driver.

Also, the datasheet states that a high precision measurement is done
before the heater is turned off. That measurement is currently ignored.

I am not entirely sure how to handle all this, but based on the information
in the datasheet I can not accept your current solution. At the very least,
you'll need to make sure that an attempt to read the humidity immediately after
turning on the heater does not result in an error message.

Thanks,
Guenter