[RFC PATCH 1/2] ftrace: Allow arches to opt-out of static ftrace
mpe at ellerman.id.au
Mon Mar 19 12:02:54 AEDT 2018
Steven Rostedt <rostedt at goodmis.org> writes:
> On Sat, 17 Mar 2018 00:46:32 +1100
> Michael Ellerman <mpe at ellerman.id.au> wrote:
>> There is a small but non-zero amount of code required by arches to
>> suppory non-dynamic (static) ftrace, and more importantly there is the
>> added work of testing both configurations.
>> There are also almost no down sides to dynamic ftrace once it's well
>> tested, other than a small increase in code/data size.
>> So give arches the option to opt-out of supporting static ftrace.
>> This is implemented as a DYNAMIC_FTRACE_CHOICE option, which controls
>> whether DYNAMIC_FTRACE is presented as a user-selectable option or if
>> it is just enabled based on its dependencies being enabled (because
>> it's already default y).
>> Then the CHOICE option depends on an arch *not* selecting
>> HAVE_DYNAMIC_FTRACE_ONLY. This would be more natural in reverse, as a
>> HAVE_STATIC_FTRACE option, but that would require updating every arch.
>> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> Why not just add in arch/powerpc/Kconfig:
> config PPC
> select DYNAMIC_FTRACE if FUNCTION_TRACER
> It seems to work for me.
It does work, but it's a bit fragile. It requires duplicating the
dependencies of DYNAMIC_FTRACE in the 'if' condition.
depends on FUNCTION_TRACER
depends on HAVE_DYNAMIC_FTRACE
So technically we should use:
select DYNAMIC_FTRACE if FUNCTION_TRACER and HAVE_DYNAMIC_FTRACE
Though we happen to know we just selected HAVE_DYNAMIC_FTRACE so we can
leave that out.
As long as the dependencies of DYNAMIC_FTRACE don't change, or the 'if'
clause is updated when they do, then it's OK and it is certainly simpler.
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