From mboxrd@z Thu Jan  1 00:00:00 1970
From: joshc@linux.com (Josh Cartwright)
Date: Fri, 6 Jan 2012 09:09:25 -0500
Subject: DEFINE Macro
In-Reply-To: <4F066B60.5000701@zoho.com>
References: <4F066B60.5000701@zoho.com>
Message-ID: <20120106140925.GF14353@joshcartwright.net>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

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