Re: [PATCH 2/4] iio: Introduce IIO_CHAN_INFO_RANGE

[Jonathan Cameron <jic23@kernel.org>]

On 23/02/15 19:35, Roberta Dobrescu wrote:
> Some ambient light sensors have hardware dependent ranges and
> resolutions. In this case user won't be able to calculate illuminance
> (lux) using only scale attribute.
> 
> For instance, a device that uses a Full Scale Range is the
> light sensor ISL29018. In this case, IIO_CHAN_INFO_RANGE is
> needed since the number of ADC bits can be 4, 8, 12 or 16 and
> using just scales would result in too similar values for different
> ranges and resolutions.
This is always an interesting corner.  The big question is what is
to be gained by ever running these sensors in their lower resolutions?

The reason I've always resisted range is that it's too easy
for people to be lazy and go with which ever of scale / range
is presented in the datasheet.  Most of the time they are really
the same thing.

The exception as you've noted here is in variable resolution devices.
There is no way that any generic code is ever going to know the right
option for the combinations of scaling and adc resolution vs time
requirements.  So we are dealing here with parameters that might
be hand tweaked for a particular board.

Anyhow, I'd like more opinions on this before I take it.

Lars, Peter, Hartmut - what you guys think on this?


> 
> Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
> ---
>  Documentation/ABI/testing/sysfs-bus-iio | 13 +++++++++++++
>  drivers/iio/industrialio-core.c         |  1 +
>  include/linux/iio/iio.h                 |  1 +
>  3 files changed, 15 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
> index 9a70c31..ad1541f 100644
> --- a/Documentation/ABI/testing/sysfs-bus-iio
> +++ b/Documentation/ABI/testing/sysfs-bus-iio
> @@ -1249,3 +1249,16 @@ Contact:	linux-iio@vger.kernel.org
>  Description:
>  		Specifies number of seconds in which we compute the steps
>  		that occur in order to decide if the consumer is making steps.
> +
> +What:		/sys/bus/iio/devices/deviceX/in_illuminance0_range
> +KernelVersion:	3.21
> +Contact:	linux-iio@vger.kernel.org
> +Description:
> +		Hardware dependent ADC Full Scale Range used for some ambient
> +		light sensors in calculating lux.
> +
> +What:		/sys/bus/iio/devices/deviceX/in_illuminance_range_available
> +KernelVersion:	3.21
> +Contact:	linux-iio@vger.kernel.org
> +Description:
> +		Hardware dependent supported values for ADC Full Scale Range.
> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> index aaba9d3..4138042 100644
> --- a/drivers/iio/industrialio-core.c
> +++ b/drivers/iio/industrialio-core.c
> @@ -128,6 +128,7 @@ static const char * const iio_chan_info_postfix[] = {
>  	[IIO_CHAN_INFO_CALIBWEIGHT] = "calibweight",
>  	[IIO_CHAN_INFO_DEBOUNCE_COUNT] = "debounce_count",
>  	[IIO_CHAN_INFO_DEBOUNCE_TIME] = "debounce_time",
> +	[IIO_CHAN_INFO_RANGE] = "range",
>  };
>  
>  /**
> diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> index 80d8550..fbdd434 100644
> --- a/include/linux/iio/iio.h
> +++ b/include/linux/iio/iio.h
> @@ -43,6 +43,7 @@ enum iio_chan_info_enum {
>  	IIO_CHAN_INFO_CALIBWEIGHT,
>  	IIO_CHAN_INFO_DEBOUNCE_COUNT,
>  	IIO_CHAN_INFO_DEBOUNCE_TIME,
> +	IIO_CHAN_INFO_RANGE,
>  };
>  
>  enum iio_shared_by {
>