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
在 2012年1月6日 上午11:32，Fredrick <fjohnber@zoho.com>写道：
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
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Oh, This is the inline ASM in gcc, pls refer to http://ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html -- ======================================= Pixelworks Room 301-303 No. 88,Lane 887 Zuchongzhi Road, Zhangjiang Hi-tech Park, Shanghai 201203, China Best Regards, Jun zhao/赵军 +++++++++++++++++++++++++++++++++++++++
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
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).
participants (3)
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Fredrick -
Josh Cartwright -
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