Messages in this thread | | | Date | Fri, 21 Nov 2008 15:38:27 +0000 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH] ftrace: mcount record based dynamic tracing for ARM |
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On Fri, Nov 21, 2008 at 09:27:17AM -0500, Steven Rostedt wrote: > On Thu, 20 Nov 2008, Jim Radford wrote: > > - select HAVE_FUNCTION_TRACER if (!XIP_KERNEL) > > + select HAVE_FTRACE_MCOUNT_RECORD > > + select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) > > Russell mentioned something about the code not being compatible with > Thumb2, is the above if statement enough?
I think I said (or should've said) "upcoming Thumb 2" - it's not yet in the kernel but there's a patch series floating around for it. We've started on merging some of the pre-requisits, and it will mean that the instruction length is no longer constant. (It may be a 16bit or 32bit instruction.)
I suspect that ftrace won't be able to handle that, so it may have to depend on !THUMB2_KERNEL for the time being.
> > diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S > > index 06269ea..06216af 100644 > > --- a/arch/arm/kernel/entry-common.S > > +++ b/arch/arm/kernel/entry-common.S > > @@ -104,14 +104,7 @@ ENDPROC(ret_from_fork) > > #ifdef CONFIG_FUNCTION_TRACER > > #ifdef CONFIG_DYNAMIC_FTRACE > > ENTRY(mcount) > > - stmdb sp!, {r0-r3, lr} > > - mov r0, lr > > - sub r0, r0, #MCOUNT_INSN_SIZE > > - > > - .globl mcount_call > > -mcount_call: > > - bl ftrace_stub > > - ldmia sp!, {r0-r3, pc} > > + mov pc, lr > > This looks OK. The new mcount stub for dynamic ftrace is a simple return. > It has been a while since I've programmed ARM (7 years ago), but I'm > assuming that loading the program counter with the link register is > a simple return.
Yes, that's all that's required.
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