linux-kernel - Re: [PATCH] ASoC: codec: wm9860: avoid maybe-uninitialized warning

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Message-ID: <CAEnQRZAkt3cz2rO9TY7nwXOLhVdCmx-V82hSr11cPfjtczWO9w@mail.gmail.com>
Date:   Thu, 20 Apr 2017 11:22:04 +0300
From:   Daniel Baluta <daniel.baluta@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Charles Keepax <ckeepax@...nsource.wolfsonmicro.com>,
        Daniel Baluta <daniel.baluta@....com>,
        patches@...nsource.wolfsonmicro.com, alsa-devel@...a-project.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ASoC: codec: wm9860: avoid maybe-uninitialized warning

On Thu, Apr 20, 2017 at 10:44 AM, Arnd Bergmann <arnd@...db.de> wrote:
> On Thu, Apr 20, 2017 at 8:48 AM, Daniel Baluta <daniel.baluta@...il.com> wrote:
>> Hi Arnd,
>>
>> On Wed, Apr 19, 2017 at 8:04 PM, Arnd Bergmann <arnd@...db.de> wrote:
>>> The new PLL configuration code triggers a harmless warning:
>>>
>>> sound/soc/codecs/wm8960.c: In function 'wm8960_configure_clocking':
>>> sound/soc/codecs/wm8960.c:735:3: error: 'best_freq_out' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>>>    wm8960_set_pll(codec, freq_in, best_freq_out);
>>>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> sound/soc/codecs/wm8960.c:699:12: note: 'best_freq_out' was declared here
>>>
>>> I think the warning was introduced by Daniel's bugfix. I've come up
>>> with a way to simplify the code in a way that is more readable to
>>> both humans and to gcc, which gets us rid of the warning.
>>
>> I've struggled with this kind of warning when reworking wm8960
>> bitclock computation.
>>
>> Anyhow, for this exact patch the warning didn't showed up.
>>
>> I used:
>>
>> gcc version 6.2.1 20161016 (Linaro GCC 6.2-2016.11)
>
> I'm using gcc-7.0.1 here, which overall has better warnings for
> -Wmaybe-uninitialized
> than older versions, but sometimes also finds new false positives.
>
>> My next patch:
>>
>> https://patchwork.kernel.org/patch/9666921/ [1]
>>
>> which is under review, had the issue you mention (in v1)
>>
>> https://lkml.org/lkml/2017/4/5/246
>>
>> but I initialized best_freq_out with 0 to get rid of the problem.
>
> I try hard not to do that, because it can hide real problems in case the
> code gets modified further to actually have an uninitialized use that we
> would otherwise get a warning for.
>
>>> @@ -720,22 +718,14 @@ int wm8960_configure_pll(struct snd_soc_codec *codec, int freq_in,
>>>                                         *sysclk_idx = i;
>>>                                         *dac_idx = j;
>>>                                         *bclk_idx = k;
>>> -                                       best_freq_out = freq_out;
>>> -                                       break;
>>> +                                       return freq_out;
>>>                                 }
>>>                         }
>>> -                       if (k != ARRAY_SIZE(bclk_divs))
>>> -                               break;
>>>                 }
>>> -               if (j != ARRAY_SIZE(dac_divs))
>>> -                       break;
>>>         }
>>> -
>>> -       if (*bclk_idx != -1)
>>> -               wm8960_set_pll(codec, freq_in, best_freq_out);
>>
>> I think the compiler is tricked into thinking that best_freq_out might
>> be uninitialized. Notice
>> that each time *bclk_idx is assigned a value (which is always >=0) we
>> also initialize best_freq_out.
>
> Right. This is probably the result of one of two things that
> prevent the compiler from figuring it out:
>
> a) bclk_idx being an indirect pointer might cause the compiler to
>     decide that another operation might overwrite it, e.g. if it points
>     to the same location as one of the other pointers.
>
> b) The flow analysis might just get too tricky, so when gcc gives
>     up trying to work out whether the assignment has happened
>     at least once for the two variables, it concludes that it doesn't
>     know, without seeing that the answer is always the same for
>     both of them.
>
>>> @@ -783,11 +773,12 @@ static int wm8960_configure_clocking(struct snd_soc_codec *codec)
>>>                 }
>>>         }
>>>
>>> -       ret = wm8960_configure_pll(codec, freq_in, &i, &j, &k);
>>> -       if (ret < 0) {
>>> +       freq_out = wm8960_configure_pll(codec, freq_in, &i, &j, &k);
>>> +       if (freq_out < 0) {
>>>                 dev_err(codec->dev, "failed to configure clock via PLL\n");
>>> -               return -EINVAL;
>>> +               return freq_out;
>>>         }
>>> +       wm8960_set_pll(codec, freq_in, freq_out);
>>>
>>>  configure_clock:
>>>         /* configure sysclk clock */
>>
>> Your idea looks good, will need to rework my follow up patch on it:
>>
>> https://patchwork.kernel.org/patch/9666921/
>
> Ok. Or feel free to fold my patch into yours if that makes it easier for you.

Great! I will fold your patch into my changes.

thanks Arnd!

Daniel.