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Subject: Re: [PATCH v6 05/13] iio: afe: rescale: add INT_PLUS_{MICRO,NANO}
 support
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
References: <20210721030613.3105327-1-liambeguin@gmail.com>
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From:   Peter Rosin <peda@axentia.se>
Organization: Axentia Technologies AB
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Date:   Fri, 30 Jul 2021 08:49:57 +0200
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On 2021-07-29 17:56, Liam Beguin wrote:
> On Wed Jul 28, 2021 at 3:19 AM EDT, Peter Rosin wrote:
>> On 2021-07-28 02:21, Liam Beguin wrote:
>>> On Fri Jul 23, 2021 at 5:16 PM EDT, Peter Rosin wrote:
>>>> On 2021-07-21 05:06, 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 | 14 ++++++++++++++
>>>>>  1 file changed, 14 insertions(+)
>>>>>
>>>>> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
>>>>> index d0669fd8eac5..2b73047365cc 100644
>>>>> --- a/drivers/iio/afe/iio-rescale.c
>>>>> +++ b/drivers/iio/afe/iio-rescale.c
>>>>> @@ -41,6 +41,20 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
>>>>>  		do_div(tmp, 1000000000LL);
>>>>>  		*val = tmp;
>>>>>  		return scale_type;
>>>>> +	case IIO_VAL_INT_PLUS_NANO:
>>>>> +		tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
>>>>> +		tmp = div_s64(tmp, rescale->denominator);
>>>>> +
>>>>> +		*val = div_s64(tmp, 1000000000LL);
>>>>> +		*val2 = tmp - *val * 1000000000LL;
>>>>> +		return scale_type;
>>>
>>> Hi Peter,
>>>
>>>>
>>>> Hi!
>>>>
>>>> My objection from v5 still stands. Did you forget or did you simply send
>>>> the
>>>> wrong patch?
>>>
>>> Apologies, again I didn't mean to make it seem like I ignored your comments.
>>> I tried your suggestion, but had issues when *val2 would overflow into
>>> the integer part.
> 
> Hi Peter,
> 
>>
>> Not saying anything about it not working does indeed make it seem like
>> you
>> ignored it :-) Or did I just miss where you said this? Anyway, no
>> problem,
>> it can be a mess dealing with a string of commits when there are
>> numerous
>> things to take care of between each iteration. And it's very easy to
>> burn
>> out and just back away. Please don't do that!
> 
> It was my mistake. Thanks for the encouragement :-)
> 
>>
>>> Even though what I has was more prone to integer overflow with the first
>>> multiplication, I thought it was still a valid solution as it passed the
>>> tests.
>>
>> I did state that you'd need to add overflow handling from the fraction
>> calculation and handling for negative values, so it was no surprise that
>> my original sketchy suggestion didn't work as-is.
>>
>>>
>>>>
>>>> Untested suggestion, this time handling negative values and
>>>> canonicalizing any
>>>> overflow from the fraction calculation.
>>>>
>>>> neg = *val < 0 || *val2 < 0;
>>>> tmp = (s64)abs(*val) * rescale->numerator;
>>>> rem = do_div(tmp, rescale->denominator);
>>>> *val = tmp;
>>>> tmp = rem * 1000000000LL + (s64)abs(*val2) * rescale->numerator;
>>>> do_div(tmp, rescale->denominator);
>>>> *val2 = do_div(tmp, 1000000000LL);
>>>> *val += tmp;
>>>> if (neg) {
>>>> if (*val < 0)
>>>> *val = -*val;
>>>> else
>>>> *val2 = -*val;
>>
>> This last line should of course be *val2 = -*val2;
>> Sorry.
>>
>>>
>>> I'll look into this suggestion.
>>
>> Thanks!
>>
> 
> Starting from what you suggested, here's what I came up with.
> I also added a few test cases to cover corner cases.
> 
> 	if (scale_type == IIO_VAL_INT_PLUS_NANO)
> 		mult = 1000000000LL;
> 	else
> 		mult = 1000000LL;
> 	/*
> 	 * For IIO_VAL_INT_PLUS_{MICRO,NANO} scale types if *val OR
> 	 * *val2 is negative the schan scale is negative
> 	 */
> 	neg = *val < 0 || *val2 < 0;
> 
> 	tmp = (s64)abs(*val) * (s32)abs(rescale->numerator);

Small nit, but I think abs() returns a signed type compatible
with the argument type. I.e. (s32)abs(rescale->...) where both
numerator and denominator are already s32 could just as well
be written without the cast as plain old abs(rescale->...)


> 	*val = div_s64_rem(tmp, (s32)abs(rescale->denominator), &rem);
> 
> 	tmp = (s64)rem * mult +
> 		(s64)abs(*val2) * (s32)abs(rescale->numerator);
> 	tmp = div_s64(tmp, (s32)abs(rescale->denominator));
> 
> 	*val += div_s64_rem(tmp, mult, val2);
> 
> 	/*
> 	 * If the schan scale or only one of the rescaler elements is
> 	 * negative, the combined scale is negative.
> 	 */
> 	if (neg || ((rescale->numerator < 0) ^ (rescale->denominator < 0)))
> 		*val = -*val;

Unconditionally negating *val doesn't negate the combined value when
*val is zero and *val2 isn't. My test "if (*val < 0)" above attempting
to take care of this case is clearly not right. It should of course be
"if (*val > 0)" since *val is not yet negated. Duh!

In fact, I think a few tests scaling to/from the [-1,1] interval
would be benefitial for this exact reason.

Cheers,
Peter