linux-kernel - Re: [PATCH v4 2/3] iio: add MEMSensing MSA311 3-axis accelerometer driver

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Date:   Fri, 5 Aug 2022 14:19:21 +0000
From:   Dmitry Rokosov <DDRokosov@...rdevices.ru>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "stano.jakubek@...il.com" <stano.jakubek@...il.com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "jic23@...nel.org" <jic23@...nel.org>,
        "lars@...afoo.de" <lars@...afoo.de>,
        "stephan@...hold.net" <stephan@...hold.net>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        kernel <kernel@...rdevices.ru>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/3] iio: add MEMSensing MSA311 3-axis accelerometer
 driver

On Thu, Aug 04, 2022 at 08:28:28PM +0200, Andy Shevchenko wrote:
> > > > +static const struct {
> > > > +       int val;
> > > > +       int val2;
> > > > +} msa311_fs_table[] = {
> > > > +       {0, 9580}, {0, 19160}, {0, 38320}, {0, 76641}
> > > > +};
> > >
> > > Besides that this struct is defined not only once in the file, this is
> > > very well NIH struct s32_fract. Why not use the latter?
> 
> > Good point, but looks like this struct is not so popular in the other
> > kernel drivers:
> 
> It's simply new, it is not about popularity. I would put it as it's
> not _yet_ so popular.

Okay, no problem. I've already reworked it to s32_fract locally.

> 
> ...
> 
> > grep "s32_fract" -r -l . | wc -l
> > 3
> 
> Hint: `git grep` much much faster on Git repositories (it goes via
> index and not working copy) and see
> `git grep -lw s32_fract`
> 

Thank you for the hint. Actually I'm using ripgrep for the kernel
fuzzysearching, it's faster than git grep. Here I gave an example based
on grep command, because it's general shell command for the searching
substrings I suppose.

> ...
> 
> > > > +       .cache_type = REGCACHE_RBTREE,
> > >
> > > Tree hash is good for sparse data, do you really have it sparse (a lot
> > > of  big gaps in between)?
> >
> > I suppose so. MSA311 regmap has ~6 gaps.
> 
> Yes and how long is each gap in comparison to the overall space?

The longest gap is 0xb bytes.
> 
> ...
> 
> > > > +       for (fs = 0; fs < ARRAY_SIZE(msa311_fs_table); ++fs)
> > >
> > > fs++ will work as well.
> >
> > I would prefer ++fs if you don't mind :)
> 
> Why? It's a non-standard pattern, and needs an explanation.
> 

>From statistics point of view, you are right :)

$ rg "for" | rg "\+\+[a-z]" | wc -l
7271
$ rg "for" | rg "[a-z]\+\+" | wc -l
81247

[...]

> > > > +                               dev_err(dev, "cannot update freq (%d)\n", err);
> > >
> > > frequency
> >
> > It will be more ugly due 80 symbols restriction.
> 
> Nope, it will be understandable by the user. You do code for the user
> and then for the developer, right?

Sure, that's good point.

[...]

> 
> ...
> 
> > > > +               dev_dbg(dev, "found MSA311 compatible chip[%#x]\n", partid);
> > >
> > > Useless message.
> >
> > Why? It's under dynamic debug, so I will see it if I really want to.
> 
> So what the point of the _successful_ detection? You already know from
> the code, that partid, you know by other means that probe was
> successful, why this message is needed? Especially for debugging when
> we have initcall_debug option.
> 

Agreed

[...]

-- 
Thank you,
Dmitry