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Documentation: CodingStyle: add inline assembly guidelines

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Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Josh Triplett 2012-03-30 13:37:10 -07:00 committed by Linus Torvalds
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@ -793,6 +793,35 @@ own custom mode, or may have some other magic method for making indentation
work correctly.
Chapter 19: Inline assembly
In architecture-specific code, you may need to use inline assembly to interface
with CPU or platform functionality. Don't hesitate to do so when necessary.
However, don't use inline assembly gratuitously when C can do the job. You can
and should poke hardware from C when possible.
Consider writing simple helper functions that wrap common bits of inline
assembly, rather than repeatedly writing them with slight variations. Remember
that inline assembly can use C parameters.
Large, non-trivial assembly functions should go in .S files, with corresponding
C prototypes defined in C header files. The C prototypes for assembly
functions should use "asmlinkage".
You may need to mark your asm statement as volatile, to prevent GCC from
removing it if GCC doesn't notice any side effects. You don't always need to
do so, though, and doing so unnecessarily can limit optimization.
When writing a single inline assembly statement containing multiple
instructions, put each instruction on a separate line in a separate quoted
string, and end each string except the last with \n\t to properly indent the
next instruction in the assembly output:
asm ("magic %reg1, #42\n\t"
"more_magic %reg2, %reg3"
: /* outputs */ : /* inputs */ : /* clobbers */);
Appendix I: References