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authorJosh Triplett <josh@joshtriplett.org>2012-03-30 13:37:10 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2012-03-30 16:03:15 -0700
commit9a7c48b7c3d58835b3a91d86c55e0ae77d15ddd5 (patch)
tree9a7728d24c9721d7e9e5458b7f76d565265fa9ae /Documentation/CodingStyle
parent21106b0114f75cff199d6834f676877c3edae928 (diff)
downloadlinux-9a7c48b7c3d58835b3a91d86c55e0ae77d15ddd5.tar.gz
Documentation: CodingStyle: add inline assembly guidelines
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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@@ -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