linux-kernel - Re: [PATCH v4 2/2] iio: adc: ad7173: add AD7173 driver

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Date:   Mon, 20 Nov 2023 18:21:05 +0200
From:   Andy Shevchenko <andy@...nel.org>
To:     Ceclan Dumitru <mitrutzceclan@...il.com>
Cc:     linus.walleij@...aro.org, brgl@...ev.pl,
        linux-gpio@...r.kernel.org, Lars-Peter Clausen <lars@...afoo.de>,
        Jonathan Cameron <jic23@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Michael Walle <michael@...le.cc>,
        Arnd Bergmann <arnd@...db.de>,
        ChiaEn Wu <chiaen_wu@...htek.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Leonard Göhrs <l.goehrs@...gutronix.de>,
        Mike Looijmans <mike.looijmans@...ic.nl>,
        Haibo Chen <haibo.chen@....com>,
        Hugo Villeneuve <hvilleneuve@...onoff.com>,
        Ceclan Dumitru <dumitru.ceclan@...log.com>,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] iio: adc: ad7173: add AD7173 driver

On Mon, Nov 20, 2023 at 05:55:12PM +0200, Ceclan Dumitru wrote:
> On 11/20/23 15:00, Andy Shevchenko wrote:
> > On Thu, Nov 16, 2023 at 03:46:55PM +0200, mitrutzceclan wrote:

...

> >> +struct ad7173_channel_config {
> >> +	bool live;
> >> +	u8 cfg_slot;
> >> +	/* Following fields are used to compare equality. Bipolar must be first */
> >> +	bool bipolar;
> >> +	bool input_buf;
> >> +	u8 odr;
> >> +	u8 ref_sel;
> > 
> > If you group better by types, it might save a few bytes on the architectures /
> > compilers where bool != byte.
> >
> Grouping by type will result in not being able to use memcmp() for
> comparing configs. But then there is the issue that I was under the
> assumption that bool=byte. If that is not the case, the config equality
> check might be comparing padding bytes.

It's most likely the case, BUT... it is not guaranteed by the C standard.

> In this case what do you suggest:
> - using the packed attribute
> - using only u8
> - drop memcmp, manually compare fields

Use struct_group() to show explicitly the group of members. If it's not an ABI
in any sense, then memcmp() is fine.

...

> >> +	cmp_size = sizeof(*cfg) - offset;
> > 
> > sizeof_field() from the above mentioned header?
> 
> This computes the size of multiple fields, following cfg_slot. Better to
> group the fields that need to be compared into another struct then use
> sizeof_field()?

See above.

...

> >> +	return vref / (MICRO/MILLI);
> > 
> > What does the denominator mean and why you can't simply use MILL?
> 
> Original vref values are in micro, I considered that it was adequate to
> represent the conversion from MICRO to MILLI as a fraction.
> 
> >> +		*val = st->info->sinc5_data_rates[reg] / (MICRO/MILLI);
> >> +		*val2 = (st->info->sinc5_data_rates[reg] % MILLI) * (MICRO/MILLI);
> > 
> > Same Q about denominator.
> > 
> Here, a misunderstanding on my part of a suggestion from Jonathan in V2,
> will be removed.

You need to clarify with him that.

-- 
With Best Regards,
Andy Shevchenko