linux-kernel - Re: [PATCH 1/6] iio: chemical: scd30: add core driver

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Message-ID: <CAHp75Vdajf7Ci3ytxP7Qs9=fFaxvVBQoL5uh+HUDwxHS5r9MUg@mail.gmail.com>
Date:   Sat, 25 Apr 2020 14:43:35 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Tomasz Duszynski <tomasz.duszynski@...akon.com>
Cc:     linux-iio <linux-iio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Jonathan Cameron <jic23@...nel.org>
Subject: Re: [PATCH 1/6] iio: chemical: scd30: add core driver

On Fri, Apr 24, 2020 at 10:05 PM Tomasz Duszynski
<tomasz.duszynski@...akon.com> wrote:
> On Wed, Apr 22, 2020 at 10:49:44PM +0300, Andy Shevchenko wrote:
> > On Wed, Apr 22, 2020 at 5:22 PM Tomasz Duszynski
> > <tomasz.duszynski@...akon.com> wrote:

...

> > > Add Sensirion SCD30 carbon dioxide core driver.
> >
> > And DocLink tar of Datasheet: with a link?
>
> I never do this. These files change their location way too often to be
> worthwhile putting here. Nobody has that much time to fallow all this
> and keep respective files up to date.
>
> But that doesn't mean I can't drop a link here.
> https://developer.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/9.5_CO2/Sensirion_CO2_Sensors_SCD30_Interface_Description.pdf

Yes, just make it a tag

DocLink: ....

...

> > > +int scd30_probe(struct device *dev, int irq, const char *name, void *priv,
> > > +               int (*command)(struct scd30_state *state, enum scd30_cmd cmd,
> > > +                              u16 arg, char *rsp, int size));
> >
> > My gosh.
> > Please, supply proper structure member in priv or alike.
>
> Not sure it's worth the fuss. Wrapping all into structure means either
> copying respective members or more dereferences later on.

At least you may introduce a typedef, because above really hurts my eyes.

...

> > > +enum {
> > > +       CONC,
> > > +       TEMP,
> > > +       HR,
> > > +};
> >
> > Way too generic names for anonymous enum.
>
> I'd argue that they are pretty well understood abbreviations in iio generally
> and here specifically. But adding some prefix won't harm.

Yes, prefix is what I was talking about.

...

> > > +static int scd30_wait_meas_poll(struct scd30_state *state)
> > > +{
> > > +       int tries = 5;
> > > +
> > > +       while (tries--) {
> > > +               int ret;
> > > +               u16 val;
> > > +
> > > +               ret = scd30_command(state, CMD_MEAS_READY, 0, (char *)&val,
> > > +                                   sizeof(val));
> > > +               if (ret)
> > > +                       return -EIO;
> > > +
> > > +               /* new measurement available */
> > > +               if (val)
> > > +                       break;
> > > +
> > > +               msleep_interruptible(state->meas_interval * 250);
> > > +       }
> > > +
> > > +       if (tries == -1)
> > > +               return -ETIMEDOUT;
> >
> > unsigned int tries = ...;
> >
> > do {
> >  ...
> > } while (--tries);
> > if (!tries)
> >   return ...;
> >
> > looks better and I guess less code in asm.
> >
>
> You mean that one extra branch in case of while?

There are few things:
a) do {} while notation immediately tells that at least one cycle of
body will be done (unconditionally);
b) it makes a loop variable unsigned and no need to check for specific
negative numbers;
c) it quite likely will generate slightly better assembly code.

>  But it comes to code
> itself it looks more compact. And I am okay with that.
>
> > > +       return 0;
> > > +}

...

> > > +       if (kstrtou16(buf, 0, &val))
> > > +               return -EINVAL;
> >
> > Shadowed error code. Don't do like this.
>
> Integer parsing either returns EINVAL or ERANGE. Passing the latter to
> the user is not worth the trouble, especially because majority of writable attrs
> have a fellow _available attr.

It's simple a bad coding practice. Please, change.

> > > +       if (kstrtou16(buf, 0, &val))
> > > +               return -EINVAL;
> >
> > Ditto.
> >
> > > +       if (kstrtou16(buf, 0, &val))
> > > +               return -EINVAL;
> >
> > Ditto.

...

> > > +       if (kstrtou16(buf, 0, &val))
> > > +               return -EINVAL;
> >
> > No shadowed error code, please. Check entire code.

Same here.

...

> > > +static IIO_DEVICE_ATTR_RW(pressure_comp, 0);
> > > +static IIO_DEVICE_ATTR_RO(pressure_comp_available, 0);
> > > +static IIO_DEVICE_ATTR_RW(meas_interval, 0);
> > > +static IIO_DEVICE_ATTR_RO(meas_interval_available, 0);
> > > +static IIO_DEVICE_ATTR_RW(asc, 0);
> > > +static IIO_DEVICE_ATTR_RW(frc, 0);
> > > +static IIO_DEVICE_ATTR_RO(frc_available, 0);
> > > +static IIO_DEVICE_ATTR_RW(temp_offset, 0);
> > > +static IIO_CONST_ATTR(temp_offset_available, "[0 1 65535]");
> > > +static IIO_DEVICE_ATTR_WO(reset, 0);
> >
> > Do you need all of them? Doesn't  IIO core provides a tons of helpers for these?
> > Btw, where is ABI documentation? It's a show stopper.
>
> They are sensor specific and none falls into a category of iio generic
> attrs. Maybe, except the measurement interval which could be represented as
> a SAMP_FREQ.

IIO ABI becomes already a big pile of nodes and I hope we will become
stricter about adding new ones.

> But given that measurement interval spans from 2s to 1800s
> it becomes a little bit awkward to have it in Hz.

> As for ABI that's in
> a separate patch.

It's not good from bisectability point of view. If by some reason this
patch or documentation patch gets reverted, the other one will be
dangling.
Please, unify them.

-- 
With Best Regards,
Andy Shevchenko