Re: [PATCH v5 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO} support

From: Peter Rosin <peda@axentia.se>
To: Liam Beguin <liambeguin@gmail.com>,
	jic23@kernel.org, lars@metafoo.de, pmeerw@pmeerw.net
Cc: linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org,
	devicetree@vger.kernel.org, robh+dt@kernel.org
Subject: Re: [PATCH v5 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO} support
Date: Sat, 17 Jul 2021 10:11:22 +0200	[thread overview]
Message-ID: <cf709fec-e2f5-5ab3-42d0-7080e11eadae@axentia.se> (raw)
In-Reply-To: <CCUT1ZDDWS1J.3CGKX5J1MNFOX@shaak>

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@xiphos.com>
>>>
>>> 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@xiphos.com>
>>> ---
>>>  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:
>>>
> 