Re: [PATCH] iio: hdc100x: correct IIO_CHAN_INFO_OFFSET value

[Jonathan Cameron <jic23@kernel.org>]

On 29/09/15 05:19, Matt Ranostay wrote:
> On Mon, Sep 28, 2015 at 2:08 AM, Jonathan Cameron
> <jic23@jic23.retrosnub.co.uk> wrote:
>>
>>
>> On 27 September 2015 23:35:29 BST, Matt Ranostay <mranostay@gmail.com> wrote:
>>> On Sun, Sep 27, 2015 at 6:29 AM, Jonathan Cameron <jic23@kernel.org>
>>> wrote:
>>>> On 27/09/15 07:18, Matt Ranostay wrote:
>>>>> Previous offset wasn't applied in the correct order and invalid.
>>>>> This patchset fixes this issue, and also has the correct scale value
>>>>> applied to the offset.
>>>>>
>>>>> Signed-off-by: Matt Ranostay <mranostay@gmail.com>
>>>> Oops, missed that.
>>>>
>>>> Given it's provided in the datasheet as effectively a fractional
>>> value
>>>> would val = -40000, val2 = 65536 and type = IIO_FRACTIONAL not be
>>> cleaner
>>>> and give the same answer?
>>>>
>>> Actually not because the way the datasheet states it  ((value / 2**16)
>>> * 165) - 40,   so the scale value needs to be applied to the -40
>>> offset (165/65536)
>> Good point.  Can't we do (-40*165)/65536 ?
> 
> Nope.
> 
> scale = 165<<2 / 65536
> offset = 40 / scale
> 
> Maybe we should add a new IIO_CHAN_INFO_SCALED_OFFSET that uses the
> scale value and a divisor that returns an IIO_FRACTIONAL?
Hmm. Would get messy fast.

Lets just go with your original fixed point option below.  If someone later figures
out a neater way of doing it, good for them ;)

J
> 
> 
>>>
>>>> Speaking of which, for the scale are we loosing any precision by
>>> shifting
>>>> the bottom of the fraction right 2 rather than the top left 2 which
>>> would have
>>>> the same result?
>>>
>>> Ah possibly. But I suspect isn't anything measurable...
>>>
>>>>
>>>> Jonathan
>>>>> ---
>>>>>  drivers/iio/humidity/hdc100x.c | 5 +++--
>>>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/iio/humidity/hdc100x.c
>>> b/drivers/iio/humidity/hdc100x.c
>>>>> index 2824578..a7f61e8 100644
>>>>> --- a/drivers/iio/humidity/hdc100x.c
>>>>> +++ b/drivers/iio/humidity/hdc100x.c
>>>>> @@ -221,8 +221,9 @@ static int hdc100x_read_raw(struct iio_dev
>>> *indio_dev,
>>>>>               }
>>>>>               break;
>>>>>       case IIO_CHAN_INFO_OFFSET:
>>>>> -             *val = -40;
>>>>> -             return IIO_VAL_INT;
>>>>> +             *val = -3971;
>>>>> +             *val2 = 879096;
>>>>> +             return IIO_VAL_INT_PLUS_MICRO;
>>>>>       default:
>>>>>               return -EINVAL;
>>>>>       }
>>>>>
>>>>
>>
>> --
>> Sent from my Android device with K-9 Mail. Please excuse my brevity.