linux-kernel - Re: [PATCH 6/6] ARM: ftrace: enable function graph tracer

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Message-ID: <20101108091858.GP16938@pengutronix.de>
Date:	Mon, 8 Nov 2010 10:18:58 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Catalin Marinas <catalin.marinas@...il.com>
Cc:	Rabin Vincent <rabin@....in>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Tim Bird <tim.bird@...sony.com>
Subject: Re: [PATCH 6/6] ARM: ftrace: enable function graph tracer

On Sat, Nov 06, 2010 at 07:06:28PM +0000, Catalin Marinas wrote:
> On Saturday, November 6, 2010, Rabin Vincent <rabin@....in> wrote:
> > Add the options to enable the function graph tracer on ARM.  Function
> > graph tracer support requires frame pointers, so exclude Thumb-2 and
> > also explicitly select FRAME_POINTER in FUNCTION_GRAPH_TRACER since
> > FUNCTION_TRACER doesn't select FRAME_POINTER when ARM_UNWIND is used
> > Therefore, with GCC 4.4.0+, you get plain function tracing without frame
> > pointers, but you'll need them if you want function graph tracing.
> .....
> > --- a/kernel/trace/Kconfig
> > +++ b/kernel/trace/Kconfig
> > @@ -141,6 +141,7 @@ config FUNCTION_TRACER
> >
> >  config FUNCTION_GRAPH_TRACER
> >         bool "Kernel Function Graph Tracer"
> > +       select FRAME_POINTER if ARM_UNWIND
> 
> Would it be better to modify FRAME_POINTER definition in the ARM
> Kconfig so that it is enabled automatically when FUNCTION_GRAPH_TRACER
> is enabled?
I don't like that suggestion much because it hides the dependency.
Moreover a "default y if FUNCTION_GRAPH_TRACER" doesn't work if a
precondition for FRAME_POINTER isn't true.  Rabin's approach at least
generates a warning.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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