DEFINE Macro

DEFINE Macro

[Fredrick]

Hi,

I am not able to understand the DEFINE macro used in
arch/powerpc/kernel/asm-offsets.c

I suppose the DEFINE is present in
include/linux/kbuild.h
where it says
#define DEFINE(sym, val) \
         asm volatile("\n->" #sym " %0 " #val : : "i" (val))

What does the above mean?


-Fredrick

DEFINE Macro

[mypopy at gmail.com]

? 2012?1?6? ??11:32?Fredrick <fjohnber@zoho.com>???

> Hi,
>
> I am not able to understand the DEFINE macro used in
> arch/powerpc/kernel/asm-offsets.c
>
> I suppose the DEFINE is present in
> include/linux/kbuild.h
> where it says
> #define DEFINE(sym, val) \
>         asm volatile("\n->" #sym " %0 " #val : : "i" (val))
>
> What does the above mean?
>
>
> -Fredrick
>
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>


Oh, This is the inline ASM in gcc, pls refer to
http://ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html

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DEFINE Macro

[Josh Cartwright]

On Thu, Jan 05, 2012 at 07:32:48PM -0800, Fredrick wrote:
> Hi,
> 
> I am not able to understand the DEFINE macro used in
> arch/powerpc/kernel/asm-offsets.c
> 
> I suppose the DEFINE is present in
> include/linux/kbuild.h
> where it says
> #define DEFINE(sym, val) \
>          asm volatile("\n->" #sym " %0 " #val : : "i" (val))
> 
> What does the above mean?

This is just a trick to get the offsets of members into a generated header file
asm-offsets.h.  The inline assembly does NOT contain valid instructions,
and in fact, asm-offsets.c is never actually assembled into a program.
Instead, the build process generates the assembly language output
asm-offsets.s, and processes it with a sed script to generate
asm-offsets.h.

For example (assume offsetof(struct thread_struct, regs) is 30):

	DEFINE(PT_REGS, offsetof(struct thread_struct, regs));

will generate within the assembly language output:

	->PT_REGS $30 offsetof(struct thread_struct, regs)

A sed script, executed on the assembly language output will generate a
line in include/generated/asm-offsets.h:

	#define PT_REGS 30 /* offsetof(struct thread_struct, regs) */

Thats about it.  You can find the exact sed script used, and the make
magic involved in Kbuild (see cmd_offsets).

-- 
                                    joshc

DEFINE Macro

[Fredrick]

Nice! Thanks for the explanation Josh.

-Fredrick

On 01/06/2012 06:09 AM, Josh Cartwright wrote:
> On Thu, Jan 05, 2012 at 07:32:48PM -0800, Fredrick wrote:
>> Hi,
>>
>> I am not able to understand the DEFINE macro used in
>> arch/powerpc/kernel/asm-offsets.c
>>
>> I suppose the DEFINE is present in
>> include/linux/kbuild.h
>> where it says
>> #define DEFINE(sym, val) \
>>           asm volatile("\n->" #sym " %0 " #val : : "i" (val))
>>
>> What does the above mean?
>
> This is just a trick to get the offsets of members into a generated header file
> asm-offsets.h.  The inline assembly does NOT contain valid instructions,
> and in fact, asm-offsets.c is never actually assembled into a program.
> Instead, the build process generates the assembly language output
> asm-offsets.s, and processes it with a sed script to generate
> asm-offsets.h.
>
> For example (assume offsetof(struct thread_struct, regs) is 30):
>
> 	DEFINE(PT_REGS, offsetof(struct thread_struct, regs));
>
> will generate within the assembly language output:
>
> 	->PT_REGS $30 offsetof(struct thread_struct, regs)
>
> A sed script, executed on the assembly language output will generate a
> line in include/generated/asm-offsets.h:
>
> 	#define PT_REGS 30 /* offsetof(struct thread_struct, regs) */
>
> Thats about it.  You can find the exact sed script used, and the make
> magic involved in Kbuild (see cmd_offsets).
>