On 2021-07-16 21:18, Liam Beguin wrote:
> On Thu Jul 15, 2021 at 5:48 AM EDT, Peter Rosin wrote:
>>
>> On 2021-07-15 05:12, Liam Beguin wrote:
>>> From: Liam Beguin <lvb@xxxxxxxxxx>
>>>
>>> Some ADCs use IIO_VAL_INT_PLUS_{NANO,MICRO} scale types.
>>> Add support for these to allow using the iio-rescaler with them.
>>>
>>> Signed-off-by: Liam Beguin <lvb@xxxxxxxxxx>
>>> ---
>>> drivers/iio/afe/iio-rescale.c | 15 +++++++++++++++
>>> 1 file changed, 15 insertions(+)
>>>
>>> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
>>> index 4c3cfd4d5181..a2b220b5ba86 100644
>>> --- a/drivers/iio/afe/iio-rescale.c
>>> +++ b/drivers/iio/afe/iio-rescale.c
>>> @@ -92,7 +92,22 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
>>> do_div(tmp, 1000000000LL);
>>> *val = tmp;
>>> return ret;
>>> + case IIO_VAL_INT_PLUS_NANO:
>>> + tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
>>> + do_div(tmp, rescale->denominator);
>>> +
>>> + *val = div_s64(tmp, 1000000000LL);
>>> + *val2 = tmp - *val * 1000000000LL;
>>> + return ret;
>>
>> This is too simplistic and prone to overflow. We need something like
>> this
>> (untested)
>>
>> tmp = (s64)*val * rescale->numerator;
>> rem = do_div(tmp, rescale->denominator);
>> *val = tmp;
>> tmp = ((s64)rem * 1000000000LL + (s64)*val2) * rescale->numerator;
>> do_div(tmp, rescale->denominator);
>> *val2 = tmp;
>>
>> Still not very safe with numerator and denominator both "large", but
>> much
>> better. And then we need normalizing the fraction part after the above,
>> of
>> course.
>>
>
> Understood, I'll test that.
I made a thinko. The remainder should not be re-multiplied with the
numerator...
tmp = (s64)*val * rescale->numerator;
rem = do_div(tmp, rescale->denominator);
*val = tmp;
tmp = (s64)rem * 1000000000LL + (s64)*val2 * rescale->numerator;
do_div(tmp, rescale->denominator);
*val2 = tmp;
And that actually reduces the risk of overflow too, which is nice!
Cheers,
Peter
>> And, of course, I'm not sure what *val == -1 and *val2 == 500000000
>> really
>> means. Is that -1.5 or -0.5? The above may very well need adjusting for
>> negative values...
>>
>
> I would've assumed the correct answer is -1 + 500000000e-9 = -0.5
> but adding a test case to iio-test-format.c seems to return -1.5...
>
> I believe that's a bug but we can work around if for now by moving the
> integer part of *val2 to *val.
>
> Liam
>
>> Cheers,
>> Peter
>>
>>> + case IIO_VAL_INT_PLUS_MICRO:
>>> + tmp = ((s64)*val * 1000000LL + *val2) * rescale->numerator;
>>> + do_div(tmp, rescale->denominator);
>>> +
>>> + *val = div_s64(tmp, 1000000LL);
>>> + *val2 = tmp - *val * 1000000LL;
>>> + return ret;
>>> default:
>>> + dev_err(&indio_dev->dev, "unsupported type %d\n", ret);
>>> return -EOPNOTSUPP;
>>> }
>>> default:
>>>
>
