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Date:   Mon, 6 Mar 2023 13:56:15 +0100
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From:   Mike Looijmans <mike.looijmans@topic.nl>
Subject: Re: [PATCH v3 2/2] iio: adc: Add TI ADS1100 and ADS1000
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To:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Jonathan Cameron <jic23@kernel.org>
CC:     devicetree@vger.kernel.org, linux-iio@vger.kernel.org,
        Caleb Connolly <caleb.connolly@linaro.org>,
        ChiYuan Huang <cy_huang@richtek.com>,
        ChiaEn Wu <chiaen_wu@richtek.com>,
        Cosmin Tanislav <demonsingur@gmail.com>,
        Ibrahim Tilki <Ibrahim.Tilki@analog.com>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Ramona Bolboaca <ramona.bolboaca@analog.com>,
        William Breathitt Gray <william.gray@linaro.org>,
        linux-kernel@vger.kernel.org
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Met vriendelijke groet / kind regards,=0A=
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On 06-03-2023 13:11, Andy Shevchenko wrote:
> On Sat, Mar 04, 2023 at 05:57:51PM +0000, Jonathan Cameron wrote:
>> On Tue, 28 Feb 2023 07:31:51 +0100
>> Mike Looijmans <mike.looijmans@topic.nl> wrote:
> ...
>
>>> +	for (i =3D 0; i < 4; i++) {
>>> +		if (BIT(i) =3D=3D gain) {
>>> +			ads1100_set_config_bits(data, ADS1100_PGA_MASK, i);
>>> +			return 0;
>>> +		}
>>> +	}
>> Andy's suggestion of something like..
>> 	if (!gain)
>> 		return -EINVAL;
>> 	i =3D ffs(gain);
>> 	if (i >=3D 4 || BIT(i) !=3D gain)
>> 		return -EINVAL;
>>
>> 	ads...
>>
>> Is perhaps nicer than the loop.
> Even better:
>
> 	if (!gain || !is_power_of_2(gain))
> 		return -EINVAL;
>
> 	i =3D ffs(gain);
> 	if (i >=3D 4)
> 		return -EINVAL;
>
I'd guess that "is_power_of_2" is about the same complexity as "ffs".

If we want smaller code, in retrospect, I'd vote for omitting that=20
power-of-two check altogether.

The IIO device reports this for a 3v3 Vdd:
# cat /sys/bus/iio/devices/iio\:device1/scale_available
0.100708007 0.050354003 0.025177001 0.012588500
# echo 0.012588500 > /sys/bus/iio/devices/iio:device1/scale

That last statement results in val2=3D12588 in this call.
The whole point of this exercise is that the value '0.012588'=20
corresponds to a gain of '8' here. There's already quite a bit of=20
rounding going on, since writing to the scale turns the value into=20
"micro" scale.

Also, looking at the "avail" table. the values for "7" and "8" are much=20
closer together than "3" and "4".

And the correct formula for the inverse of "gain =3D BIT(i);" is "i =3D=20
ffs(gain) - 1;" because ffs(1) =3D=3D 1

So I propose this code:

 =C2=A0=C2=A0=C2=A0 if (gain <=3D 0 || gain > 8)
 =C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0=C2=A0 return -EINVAL;

 =C2=A0=C2=A0 =C2=A0regval =3D ffs(gain) - 1;
 =C2=A0=C2=A0=C2=A0 ads1100_set_config_bits(data, ADS1100_PGA_MASK, regval)=
;


--=20
Mike Looijmans