linux-kernel - Re: [PATCH v9 00/14] iio: afe: add temperature rescaling support

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Date:   Sun, 28 Nov 2021 10:17:50 +0100
From:   Peter Rosin <peda@...ntia.se>
To:     Liam Beguin <liambeguin@...il.com>
Cc:     jic23@...nel.org, lars@...afoo.de, linux-kernel@...r.kernel.org,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        robh+dt@...nel.org
Subject: Re: [PATCH v9 00/14] iio: afe: add temperature rescaling support

Hi!

On 2021-11-27 21:27, Liam Beguin wrote:
> Hi Peter,
> 
> On Mon, Nov 22, 2021 at 01:53:44AM +0100, Peter Rosin wrote:
>> Hi Liam!
>>
>> On 2021-11-15 04:43, Liam Beguin wrote:
>>> Hi Jonathan, Peter,

snip

>>> - keep IIO_VAL_FRACTIONAL scale when possible, if not default to fixed
>>>   point
>>
>> This is not what is going on. Patch 9/14 will convert all fractional
>> scales to fixed point. But I would really like if you in the "reduce
>> risk of integer overflow" patch (8/14) would hold true to the above
>> and keep the fractional scale when possible and only fall back to
>> the less precise fractional-log case if any of the multiplications
>> needed for an exact fractional scale causes overflow.
> 
> Thanks for looking at these patches again.
> 
>> The v8 discussion concluded that this was a valid approach, right?
> 
> Yes, I remember you saying that you'd be more comfortable keeping the
> IIO_VAL_FRACTIONAL.
> 
>> I know you also said that the core exposes the scale with nano
>> precision in sysfs anyway, but that is not true for in-kernel
>> consumers. They have an easier time reading the "real" scale value
>> compared to going via the string representation of fixed point
>> returned from iio_format_value. At least the rescaler itself does so,
>> which means that chaining rescalers might suffer needless accuracy
>> degradation.
> 
> Agreed, that makes total sense.
> 
> If I'm not mistaken, the first condition in the case, if (!rem), will
> return IIO_VAL_FRACTIONAL if the division is exact, keeping all the
> precision. No?

Only if the resulting scale fits in nine decimals. That's never the
case if you have primes other than 2 and 5 in the denominator (after
eliminating gcd of course). Which mean that if you chain one rescaler
doing 1/3 and one doing 3/1, you would get a combined scale of
0.999999999 instead of 3/3 if we take the approach of these patches.

So, what I'm after is that - for IIO_VAL_FRACTIONAL - not take the
multiply-by-1e9 code path /unless/ the existing fractional approach
overflows in either numerator or denominator (or both).

Side note: The same could be done for IIO_VAL_INT when the numerator
overflows (since the denominator cannot overflow), but I guess that
can be done later.

Cheers,
Peter