linux-kernel - Re: [PATCH 01/16] arch: x86: boot: Make function static and add function prototype

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Date:	Wed, 4 Mar 2015 19:50:44 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Darshana Padmadas <darshanapadmadas@...il.com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, josh@...htriplett.org,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 01/16] arch: x86: boot: Make function static and add
 function prototype


* Darshana Padmadas <darshanapadmadas@...il.com> wrote:

> compressed/eboot.c defines an internal function
> setup_graphics(struct boot_params *boot_params). No
> other file refers to this function with the same
> parameters so make this function static.
> 
> This eliminates the following warning:
> 
> arch/x86/boot/compressed/eboot.c:1004:6: warning: no previous prototype for ‘setup_graphics’ [-Wmissing-prototypes]
> 
> Also make_boot_params is declared only in this file and is used
> in some assembly. So add a prototype for this function.
> 
> This eliminates the following warning:
> 
> arch/x86/boot/compressed/eboot.c:1042:21: warning: no previous prototype for ‘make_boot_params’ [-Wmissing-prototypes]
> 
> Signed-off-by: Darshana Padmadas <darshanapadmadas@...il.com>
> ---
>  arch/x86/boot/compressed/eboot.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/boot/compressed/eboot.c b/arch/x86/boot/compressed/eboot.c
> index 92b9a5f..cb4ebab 100644
> --- a/arch/x86/boot/compressed/eboot.c
> +++ b/arch/x86/boot/compressed/eboot.c
> @@ -1001,7 +1001,7 @@ free_handle:
>  	return status;
>  }
>  
> -void setup_graphics(struct boot_params *boot_params)
> +static void setup_graphics(struct boot_params *boot_params)

This category of change is good: making needlessly global functions 
static is absolutely useful even if the kernel doesn't use the 
-Wmissing-prototypes warning, because this allows the compiler to 
potentially inline the function and thus reduce the size of the 
kernel. (even if it's not inlined, it's a small reduction in kernel 
image size.)

>  {
>  	efi_guid_t graphics_proto = EFI_GRAPHICS_OUTPUT_PROTOCOL_GUID;
>  	struct screen_info *si;
> @@ -1039,6 +1039,9 @@ void setup_graphics(struct boot_params *boot_params)
>   * The caller is responsible for filling out ->code32_start in the
>   * returned boot_params.
>   */
> +
> +struct boot_params *make_boot_params(struct efi_config *c);
> +
>  struct boot_params *make_boot_params(struct efi_config *c)

This category of change is not so useful in that form: look at the 
duplicated line in the .c file, it looks weird and not very useful at 
first glance already.

The way this is typically solved in user-space projects that use -Wall 
-Werror (such as tools/perf/) is to stick the external prototypes into 
a proper header file.

If a function is global because it's only interfacing with assembly 
code then put that into a comment before the prototype lines as well, 
in the header file. In this case the extra prototypes are also useful: 
they keep people modifying those functions informed and careful, as 
changing the function parameters signature could break assembly code 
without creating any obvious build time errors or warnings.

Thanks,

	Ingo
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