linux-kernel - Re: [PATCH] leds: lm355x: avoid enum conversion warning

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Date:   Wed, 6 May 2020 16:19:45 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Pavel Machek <pavel@....cz>, Bryan Wu <bryan.wu@...onical.com>,
        "G.Shark Jeong" <gshark.jeong@...il.com>,
        Dan Murphy <dmurphy@...com>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-leds@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] leds: lm355x: avoid enum conversion warning

On Wed, May 6, 2020 at 4:44 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> On Tue, May 05, 2020 at 04:19:17PM +0200, Arnd Bergmann wrote:
> > clang points out that doing arithmetic between diffent enums is usually
>                                                  ^ different
> > a mistake:
> >
> > drivers/leds/leds-lm355x.c:167:28: warning: bitwise operation between different enumeration types ('enum lm355x_tx2' and 'enum lm355x_ntc') [-Wenum-enum-conversion]
> >                 reg_val = pdata->pin_tx2 | pdata->ntc_pin;
> >                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
> > drivers/leds/leds-lm355x.c:178:28: warning: bitwise operation between different enumeration types ('enum lm355x_tx2' and 'enum lm355x_ntc') [-Wenum-enum-conversion]
> >                 reg_val = pdata->pin_tx2 | pdata->ntc_pin | pdata->pass_mode;
> >                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
> >
> > In this driver, it is intentional, so add a cast to hide the false-positive
>
> Not sure that I would call this a false positive. The warning is correct
> that there is a bitwise operation between different enumeration types
> but we do not care since we are just using the enumerated type for its
> integer value in lieu of a #define VAR value.

Right, I meant that the code works as intended and said "false positive"
to avoid claiming the driver is broken when this was a deliberate
design point.

We do want clang to warn about this though as you say, so I can
rephrase it to explain that both the driver and the compiler work
as intended but they clash in their views of how to do it ;-)

> > -             reg_val = pdata->pass_mode;
> > +             reg_val = (u32)pdata->pass_mode;
>
> Is this cast needed? I don't think there should be warning from going
> from an enumerated type to unsigned int.

This cast is not needed for warnings, I added it for consistency because
it seemed odd to cast only four of the five enums. I can remove if though
if you think it's clearer without the cast.

There may also be a different solution in completely removing the
lm355x_chip_init() function, as it was added at a time when we
were converting the last board files into devicetree, and there has
never been a board file defining lm355x_platform_data.

There is unfortunately no DT support either in it, so I assume we
could just remove the driver completely, or change it to use a
DT binding similar to
Documentation/devicetree/bindings/leds/leds-lm36*.txt

LED maintainers, any opinions on this?

     Arnd