Re: [PATCH 3/3] iio: chemical: sgp40: Implement turn_heater_off-command

[Jonathan Cameron <jic23@kernel.org>]

On Sat, 19 Sep 2026 01:36:45 +0200
Andreas Klinger <ak@it-klinger.de> wrote:

> Hi Jaakko,
> 
> Jaakko Koivisto <jmatko@utu.fi> schrieb am Fr, 18. Sep 16:40:
> > -Turn the heating element off and enter idle mode.
> > -Present the functionality as device attribute,
> >  'echo 1 > turn_heater_off'.  
> 
> Instead of introducing a device specific attribute couldn't this be implemented
> as standard power management operations (RUNTIME_PM_OPS)?

May not apply in this case but normally the warm up time of these sorts of
heaters are in the seconds.  No one wants that latency when they want
a measurement.  As such normal runtime pm doesn't work - it needs to
be a specific userspace opt in.

We have defined ABI for this though and this isn't it
See Documentation/ABI/testing/sysfs-bus-iio (and more in -humidity)

Jonathan

> 
> Andreas
> 
> > Saves approx. 2.5 mA compared to regular operation. The heating element
> > is automatically turned back on when measurement is performed.
> > 
> > Signed-off-by: Jaakko Koivisto <jmatko@utu.fi>
> > ---
> >  drivers/iio/chemical/sgp40.c | 28 ++++++++++++++++++++++++++++
> >  1 file changed, 28 insertions(+)
> > 
> > diff --git a/drivers/iio/chemical/sgp40.c b/drivers/iio/chemical/sgp40.c
> > index 28d5e737d1dc..a4fc5c778303 100644
> > --- a/drivers/iio/chemical/sgp40.c
> > +++ b/drivers/iio/chemical/sgp40.c
> > @@ -29,6 +29,7 @@
> >   * by writing to the out values of temp and humidityrelative.
> >   */
> >  
> > +#include "linux/device.h"
> >  #include <linux/delay.h>
> >  #include <linux/crc8.h>
> >  #include <linux/module.h>
> > @@ -259,6 +260,21 @@ static int sgp40_execute_self_test(struct sgp40_data *data)
> >  	}
> >  }
> >  
> > +static int sgp40_turn_heater_off(struct sgp40_data *data)
> > +{
> > +	int ret;
> > +	struct i2c_client *client = data->client;
> > +	struct sgp40_command turn_off = {.command = {0x36, 0x15}};
> > +
> > +	ret = i2c_master_send(client, (char*)&turn_off, sizeof(turn_off.command));
> > +	if (ret != sizeof(turn_off.command)) {
> > +		dev_err(data->dev, "i2c_master_send ret: %d, expected %zu", ret, sizeof(turn_off.command));
> > +		return -EIO;
> > +	}
> > +	msleep(1);
> > +	return 0;
> > +}
> > +
> >  static int sgp40_measure_resistance_raw(struct sgp40_data *data, u16 *resistance_raw)
> >  {
> >  	int ret;
> > @@ -417,10 +433,22 @@ static ssize_t serial_number_show(struct device *dev,
> >  	return sysfs_emit_at(buf, 0, "%llu\n", data->serial_number);
> >  }
> >  
> > +static ssize_t turn_heater_off_store(struct device *dev,
> > +				     struct device_attribute *attr,
> > +				     const char *buf, size_t len)
> > +{
> > +	struct sgp40_data *data = iio_priv(dev_to_iio_dev(dev));
> > +	sgp40_turn_heater_off(data);
> > +
> > +	return len;
> > +}
> > +
> >  static IIO_DEVICE_ATTR_RO(serial_number, 0);
> > +static IIO_DEVICE_ATTR_WO(turn_heater_off, 0);
> >  
> >  static struct attribute *sgp40_attributes[] = {
> >  	&iio_dev_attr_serial_number.dev_attr.attr,
> > +	&iio_dev_attr_turn_heater_off.dev_attr.attr,
> >  	NULL
> >  };
> >  
> > -- 
> > 2.55.0
> >   
>