linux-kernel - Re: [PATCH v6 2/2] iio: (bma400) add driver for the BMA400

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Date:   Thu, 12 Dec 2019 11:41:45 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Dan Robertson <dan@...obertson.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        linux-iio <linux-iio@...r.kernel.org>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        devicetree <devicetree@...r.kernel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v6 2/2] iio: (bma400) add driver for the BMA400

On Thu, Dec 12, 2019 at 2:33 AM Dan Robertson <dan@...obertson.com> wrote:
> On Wed, Dec 11, 2019 at 03:21:56PM +0200, Andy Shevchenko wrote:
> > On Wed, Dec 11, 2019 at 3:20 AM Dan Robertson <dan@...obertson.com> wrote:

> > > +#define BMA400_LP_OSR_SHIFT         0x05
> > > +#define BMA400_NP_OSR_SHIFT         0x04
> > > +#define BMA400_SCALE_SHIFT          0x06
> >
> > I'm not sure why this is being defined as hex number instead of plain decimal...
>
> Sounds good.
>
> > > +#define BMA400_TWO_BITS_MASK        GENMASK(1, 0)
> > > +#define BMA400_LP_OSR_MASK          GENMASK(6, BMA400_LP_OSR_SHIFT)
> > > +#define BMA400_NP_OSR_MASK          GENMASK(5, BMA400_NP_OSR_SHIFT)
> > > +#define BMA400_ACC_ODR_MASK         GENMASK(3, 0)
> > > +#define BMA400_ACC_SCALE_MASK       GENMASK(7, BMA400_SCALE_SHIFT)
> >
> > And here simple better to put same numbers. It will help to read.
>
> Do you mean for the shift or for the mask?

SHIFTs -> plain decimals

> > > +EXPORT_SYMBOL(bma400_regmap_config);
> >
> > I'm not sure I got the idea why this one is being exported.
>
> It needs to be exported so that it can be used in the bma400_i2c module and the
> future bma400_spi module. In theory, if we _really_ do not want to export this,
> then we can define separate regmap configs in each of the bma400_i2c and
> (future) bma400_spi modules, but then we would have to export the is_volitile_reg
> and is_writable_reg functions. As a result, I do not see any benefits to that
> method over exporting the config, but I could be convinced otherwise.

I think there might be better way to do this.
But I leave it to you and maintainer to agree on (I will be fine with
any solution you will come to).

> > > +               if (uhz || hz % BMA400_ACC_ODR_MIN_WHOLE_HZ)
> > > +                       return -EINVAL;
> > > +
> > > +               val = hz / BMA400_ACC_ODR_MIN_WHOLE_HZ;
> > > +               idx = __ffs(val);
> > > +
> >
> > > +               if (val ^ BIT(idx))
> >
> > Seems like funny way of checking is_power_of_2(). But it's up to maintainers.
> > And your variant may even be better here (in code generation perspective)...
> >
> > However, the whole idea here is, IIUC, to have something like
> >
> >   hz = 2^idx * BMA400_ACC_ODR_MIN_WHOLE_HZ
> >
> > I think you may do it without divisions, i.e. call __ffs() first and then do
> >    idx = __ffs(...);
> >    val = hz >> idx;
> >    if (val != BMA400_ACC_ODR_MIN_WHOLE_HZ)
> >     return -EINVAL;
> >
> > or something like above.
>
> It would be more obvious what is being done here with is_power_of_two. I'll
> revisit this function with your suggestions. If I can make it simpler, I'll
> go this route.

The main point here to get rid of divisions. Is it achievable?

> > > +                       return -EINVAL;
> >
> > ...

> > > +       ret = regmap_read(data->regmap, BMA400_ACC_CONFIG0_REG, &val);
> > > +       if (ret < 0)
> >
> > I'm wondering if in all of these regmap_read()...
> >
> > > +               return ret;
> >
> > > +       ret = regmap_write(data->regmap, BMA400_ACC_CONFIG0_REG,
> > > +                          mode | (val & ~BMA400_TWO_BITS_MASK));
> > > +       if (ret < 0) {
> >
> > ...and regmap_write() calls you ever can get a positive returned code.
>
> From the regmap_read/regmap_write docs:
>
> > * A value of zero will be returned on success, a negative errno will
> > * be returned in error cases.
>
> So I assume ret <= 0

There is no positive codes mentioned at all. And you assume right.
But why we care about positive codes if they never can be returned?

-- 
With Best Regards,
Andy Shevchenko