linux-kernel - Re: [PATCH 1/1] drm/i915/dsi: silence a warning about uninitialized return value

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Date:   Sun, 23 Oct 2016 18:55:58 +0200
From:   Nicolas Iooss <nicolas.iooss_linux@....org>
To:     Daniel Vetter <daniel.vetter@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] drm/i915/dsi: silence a warning about uninitialized
 return value

Hello,

I sent the patch below a few weeks ago. I got some comments (cf. [1])
which looked good, but the patch has not been merged in linux-next yet.
Do I need to fix something (like rewrite the commit message) in order to
get it merged?

Thanks,
Nicolas

[1] https://patchwork.freedesktop.org/patch/108941/

On 04/09/16 20:58, Nicolas Iooss wrote:
> When building the kernel with clang and some warning flags, the compiler
> reports that the return value of dcs_get_backlight() may be
> uninitialized:
> 
>     drivers/gpu/drm/i915/intel_dsi_dcs_backlight.c:53:2: error: variable
>     'data' is used uninitialized whenever 'for' loop exits because its
>     condition is false [-Werror,-Wsometimes-uninitialized]
>             for_each_dsi_port(port, intel_dsi->dcs_backlight_ports) {
>             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     drivers/gpu/drm/i915/intel_dsi.h:126:49: note: expanded from macro
>     'for_each_dsi_port'
>     #define for_each_dsi_port(__port, __ports_mask)
>                                 for_each_port_masked(__port, __ports_mask)
>                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     drivers/gpu/drm/i915/i915_drv.h:322:26: note: expanded from macro
>     'for_each_port_masked'
>         for ((__port) = PORT_A; (__port) < I915_MAX_PORTS; (__port)++)  \
>                                 ^~~~~~~~~~~~~~~~~~~~~~~~~
>     drivers/gpu/drm/i915/intel_dsi_dcs_backlight.c:60:9: note:
>     uninitialized use occurs here
>             return data;
>                    ^~~~
> 
> As intel_dsi->dcs_backlight_ports seems to be always initialized to a
> non-null value, the content of the for loop is always executed and there
> is no bug in the current code. Nevertheless the compiler has no way of
> knowing that assumption, so initialize variable 'data' to silence the
> warning here.
> 
> Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@....org>
> ---
>  drivers/gpu/drm/i915/intel_dsi_dcs_backlight.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dsi_dcs_backlight.c b/drivers/gpu/drm/i915/intel_dsi_dcs_backlight.c
> index ac7c6020c443..eec45856f910 100644
> --- a/drivers/gpu/drm/i915/intel_dsi_dcs_backlight.c
> +++ b/drivers/gpu/drm/i915/intel_dsi_dcs_backlight.c
> @@ -46,7 +46,7 @@ static u32 dcs_get_backlight(struct intel_connector *connector)
>  	struct intel_encoder *encoder = connector->encoder;
>  	struct intel_dsi *intel_dsi = enc_to_intel_dsi(&encoder->base);
>  	struct mipi_dsi_device *dsi_device;
> -	u8 data;
> +	u8 data = 0;
>  	enum port port;
>  
>  	/* FIXME: Need to take care of 16 bit brightness level */
>