Date:   Sat, 4 Mar 2023 17:26:18 +0000
From:   Jonathan Cameron <jic23@kernel.org>
To:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc:     Mike Looijmans <mike.looijmans@topic.nl>,
        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
Subject: Re: [PATCH v3 2/2] iio: adc: Add TI ADS1100 and ADS1000
Message-ID: <20230304172618.37f448d0@jic23-huawei>
In-Reply-To: <ZACxUpzCtlrMehrA@smile.fi.intel.com>
References: <20230228063151.17598-1-mike.looijmans@topic.nl>
        <20230228063151.17598-2-mike.looijmans@topic.nl>
        <Y/9vez/fzLD5dRVF@smile.fi.intel.com>
        <1b153bce-a66a-45ee-a5c6-963ea6fb1c82.949ef384-8293-46b8-903f-40a477c056ae.0685d97e-4a28-499e-a9e3-3bafec126832@emailsignatures365.codetwo.com>
        <a2ba706f-888b-0a72-03a5-cbf761dfaf19@topic.nl>
        <ZACxUpzCtlrMehrA@smile.fi.intel.com>
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On Thu, 2 Mar 2023 16:23:14 +0200
Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:

> On Thu, Mar 02, 2023 at 08:49:22AM +0100, Mike Looijmans wrote:
> > On 01-03-2023 16:30, Andy Shevchenko wrote: =20
> > > On Tue, Feb 28, 2023 at 07:31:51AM +0100, Mike Looijmans wrote: =20
>=20
> ...
>=20
> > > > +	/* Shift result to compensate for bit resolution vs. sample rate =
*/
> > > > +	value <<=3D 16 - ads1100_data_bits(data);
> > > > +	*val =3D sign_extend32(value, 15); =20
> > > Why not simply
> > >=20
> > > 	*val =3D sign_extend32(value, ads1100_data_bits(data) - 1);
> > >=20
> > > ? =20
> >=20
> > As discussed with=C2=A0 Jonathan Cameron, the register is right-justifi=
ed and the
> > number of bits depend on the data rate. Rather than having the "scale"
> > change when the sample rate changes, we chose to adjust the sample resu=
lt so
> > it's always left-justified. =20
>=20
> Hmm... OK, but it adds unneeded code I think.

There isn't a way to do it in one go that I can think of.
The first statement is multiplying the value by a power of 2, not just sign=
 extending it.
You could sign extend first then shift to do the multiply, but ends up same=
 amount
of code.

It does look a bit like a weird open coded sign extension though so I can s=
ee where
the confusion came from!

>=20
> ...
>=20
> > > > +	for (i =3D 0; i < 4; i++) {
> > > > +		if (BIT(i) =3D=3D gain) { =20
> > > ffs()/__ffs() (look at the documentation for the difference and use p=
roper one). =20
> >=20
> > Thought of it, but I'd rather have it return EINVAL for attempting to s=
et
> > the analog gain to "7" (0nly 1,2,4,8 allowed). =20
>=20
> I'm not sure what you are implying.
>=20
> You have open coded something that has already to be a function which on =
some
> architectures become a single assembly instruction.
>=20
> That said, drop your for-loop if-cond and use one of the proposed directl=
y.
> Then you may compare the result to what ever you want to be a limit and r=
eturn
> whatever error code you want to

Agreed, could do it with appropriate ffs() followed by if (BIT(i) !=3D gain=
) return -EINVAL;