linux-kernel - Re: [PATCH v2 2/3] iio: pressure: Support ROHM BU1390

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Date:   Fri, 22 Sep 2023 09:07:42 +0300
From:   Matti Vaittinen <mazziesaccount@...il.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        ak@...klinger.de, andriy.shevchenko@...ux.intel.com,
        ang.iglesiasg@...il.com, bbara93@...il.com, conor+dt@...nel.org,
        devicetree@...r.kernel.org, jic23@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, lars@...afoo.de,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        matti.vaittinen@...rohmeurope.com, robh+dt@...nel.org
Subject: Re: [PATCH v2 2/3] iio: pressure: Support ROHM BU1390

On 9/19/23 17:32, Jonathan Cameron wrote:
> 
>>>> +static int bm1390_read_raw(struct iio_dev *idev,
>>>> +               struct iio_chan_spec const *chan,
>>>> +               int *val, int *val2, long mask)
>>>> +{
>>>> +    struct bm1390_data *data = iio_priv(idev);
>>>> +    int ret;
>>>> +
>>>> +    switch (mask) {
>>>> +    case IIO_CHAN_INFO_SCALE:
>>>> +        if (chan->type == IIO_TEMP) {
>>>> +            *val = 31;
>>>> +            *val2 = 250000;
>>>> +
>>>> +            return IIO_VAL_INT_PLUS_MICRO;
>>>> +        } else if (chan->type == IIO_PRESSURE) {
>>>> +            *val = 0;
>>>> +            /*
>>>> +             * pressure in hPa is register value divided by 2048.
>>>> +             * This means kPa is 1/20480 times the register value,
>>>> +             * which equals to 48828.125 * 10 ^ -9
>>>> +             * This is 48828.125 nano kPa.
>>>> +             *
>>>> +             * When we scale this using IIO_VAL_INT_PLUS_NANO we
>>>> +             * get 48828 - which means we lose some accuracy. Well,
>>>> +             * let's try to live with that.
>>>> +             */
>>>> +            *val2 = 48828;
>>>> +
>>>> +            return IIO_VAL_INT_PLUS_NANO;
>>>> +        }
>>>> +
>>>> +        return -EINVAL;
>>>> +    case IIO_CHAN_INFO_RAW:
>>>> +        ret = iio_device_claim_direct_mode(idev);
>>>> +        if (ret)
>>>> +            return ret;
>>>> +
>>>> +        ret = bm1390_read_data(data, chan, val, val2);
>>>> +        iio_device_release_direct_mode(idev);
>>>> +        if (ret)
>>>> +            return ret;
>>>> +
>>>> +        return IIO_VAL_INT;
>>>> +    default:
>>>> +        return -EINVAL;
>>>
>>> Certainly useless, but should we break and return -EINVAL after the
>>> switch, so that it is more explicit that bm1390_read_raw() always
>>> returns a value?
>>
>> I think there is also opposite opinions on this. For my eyes the return
>> at the end of the function would also be clearer - but I think I have
>> been asked to drop the useless return when I've been working with other
>> sensors in IIO domain :) My personal preference would definitely be:
>>
>> int ret;
>>
>> switch (foo)
>> {
>> case BAR:
>> 	ret = func1();
>> 	if (ret)
>> 		break;
>>
>> 	ret = func2();
>> 	if (ret)
>> 		break;
>>
>> ...
>> 	break;
>>
>> case BAZ:
>> 	ret = -EINVAL;
>> 	break;
>> }
>>
>> return ret;
>>
>> - but I've learned to think this is not the IIO preference.
> 
> Some static analyzers get confused (probably when there is a little
> bit more going on after the function) by that and moan that some
> cases are not considered in the switch.  I got annoyed enough with the
> noise they were generating to advocate always having explicit defaults.

Oh, yes. I see I omitted the default from the example - but this was not 
what I tried to highlight ;) With a bit more thought I would've added:

default:
	ret = -EINVAL;
	break;

As you probably guess, what I was after is that for a simple (not deeply 
nested) cases like this, I would rather use a variable for return value 
and a single point of exit at the end of the function - instead of 
having returns in the switch-case. That'd suit better _my_ taste.

Yours,
	-- Matti

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~