linux-kernel - Re: [PATCH v4 2/2] iio: (bma400) add driver for the BMA400

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Message-ID: <20191123125135.4c7efcb0@archlinux>
Date:   Sat, 23 Nov 2019 12:51:35 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Dan Robertson <dan@...obertson.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        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>
Subject: Re: [PATCH v4 2/2] iio: (bma400) add driver for the BMA400

On Mon, 18 Nov 2019 00:25:04 +0000
Dan Robertson <dan@...obertson.com> wrote:

> Sorry for the incredibly late reply. Before I submit the next patchset version,
> I have a question from the last set of reviews.
> 
> On Mon, Oct 21, 2019 at 04:20:16PM +0100, Jonathan Cameron wrote:
> > On Sat, 19 Oct 2019 02:43:51 +0000
> > Dan Robertson <dan@...obertson.com> wrote:  
> > > On Fri, Oct 18, 2019 at 10:23:38AM +0300, Andy Shevchenko wrote:  
> > > > On Fri, Oct 18, 2019 at 6:44 AM Dan Robertson <dan@...obertson.com> wrote:    
> > > > > +static const int bma400_osr_table[] = { 0, 1, 3 };    
> > > >     
> > > > > +/* See the ACC_CONFIG1 section of the datasheet */
> > > > > +static const int bma400_sample_freqs[] = {
> > > > > +       12,  500000,
> > > > > +       25,  0,
> > > > > +       50,  0,
> > > > > +       100, 0,
> > > > > +       200, 0,
> > > > > +       400, 0,
> > > > > +       800, 0,
> > > > > +};    
> > > > 
> > > > This can be replaced by a formula(s).    
> > > 
> > > Yeah I think I can implement the get, set, and read functions for sample_freq
> > > with a formula, but the scale and sample frequency tables are needed by the
> > > implementation of read_avail. A implementation of read_avail with a range and
> > > a step would be ideal, but I couldn't find any documentation on implementing
> > > read_avail where the step value of the range is a multiple. Please correct
> > > me if I've missed something.  
> > 
> > Indeed. We've only defined it as being fixed intervals.
> > I'm not keen to expand the options for the userspace interface any
> > further.  
> > 
> > You could compute the values at startup and store it in your state structure
> > I think (or compute them on demand, but you'd need to have the space somewhere
> > non volatile).
> >   
> 
> I ended up writing an implementation that uses a formula for the get/set
> functions of the sample frequency and scale, but uses a table for the
> implementation of the read_avail function. While it does work, I worry
> that this makes the driver less maintainable and would make it harder to
> add support for a new hypothetical future BMA4xx device. Also, the majority
> of drivers seem to use a table for the raw value to user input conversion,
> so a move from this might make the code less "familiar".
> 
> If we do stick with the translation table, would it be better to have two
> tables (a translation table and a read_avail table) so that we do not have
> a step distance of two? This would mean we would need to maintain two
> tables, but would simplify the code.

If a function is your preferred route you could also just use it to compute
the values for the available table at startup?

Otherwise, its fine to just use a table for both.

> 
> Random workflow question:
> 
> The sampling ratio, frequency, etc code seems to be the most complicated part
> of the driver. Is it typically recommended to upstream a more minimal driver
> that might assume the defaults?

Often people upstream a first version that just uses defaults, then follow
up (if they care) with later series adding the more fiddly elements.

Sometimes those more fiddly bits never come as a particular author
never needed them.  That's absolutely fine.  It's a rare driver
that supports all the features on a non trivial device!

Thanks,

Jonathan

> 
> Cheers,
> 
>  - Dan
>