linux-kernel - Re: [PATCH] acpi: fix false-positive -Wuninitialized warning

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Date:   Fri, 12 Jul 2019 14:47:40 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Keith Busch <keith.busch@...el.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] acpi: fix false-positive -Wuninitialized warning

On Fri, Jul 12, 2019 at 11:01:21AM +0200, Arnd Bergmann wrote:
> clang gets confused by an uninitialized variable in what looks
> to it like a never executed code path:
> 
> arch/x86/kernel/acpi/boot.c:618:13: error: variable 'polarity' is uninitialized when used here [-Werror,-Wuninitialized]
>         polarity = polarity ? ACPI_ACTIVE_LOW : ACPI_ACTIVE_HIGH;
>                    ^~~~~~~~
> arch/x86/kernel/acpi/boot.c:606:32: note: initialize the variable 'polarity' to silence this warning
>         int rc, irq, trigger, polarity;
>                                       ^
>                                        = 0
> arch/x86/kernel/acpi/boot.c:617:12: error: variable 'trigger' is uninitialized when used here [-Werror,-Wuninitialized]
>         trigger = trigger ? ACPI_LEVEL_SENSITIVE : ACPI_EDGE_SENSITIVE;
>                   ^~~~~~~
> arch/x86/kernel/acpi/boot.c:606:22: note: initialize the variable 'trigger' to silence this warning
>         int rc, irq, trigger, polarity;
>                             ^
>                              = 0
> 
> This is unfortunately a design decision in clang and won't be fixed.
> 
> Changing the acpi_get_override_irq() macro to an inline function
> reliably avoids the issue.

In this particular case it looks fine (perhaps in the future -1 shall be
replaced with proper error code).

Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

But in general it looks like clang obscures use of pretty well working macros.

> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  include/linux/acpi.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/acpi.h b/include/linux/acpi.h
> index a95cce5e82e7..9426b9aaed86 100644
> --- a/include/linux/acpi.h
> +++ b/include/linux/acpi.h
> @@ -324,7 +324,10 @@ struct irq_domain *acpi_irq_create_hierarchy(unsigned int flags,
>  #ifdef CONFIG_X86_IO_APIC
>  extern int acpi_get_override_irq(u32 gsi, int *trigger, int *polarity);
>  #else
> -#define acpi_get_override_irq(gsi, trigger, polarity) (-1)
> +static inline int acpi_get_override_irq(u32 gsi, int *trigger, int *polarity)
> +{
> +	return -1;
> +}
>  #endif
>  /*
>   * This function undoes the effect of one call to acpi_register_gsi().
> -- 
> 2.20.0
> 

-- 
With Best Regards,
Andy Shevchenko