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Date: Fri, 21 Nov 2008 10:09:29 -0800
From: Jim Radford <radford@galvanix.net>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>, Sam Ravnborg <sam@ravnborg.org>,
       linux-arm-kernel@lists.arm.linux.org.uk,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] ftrace: mcount record based dynamic tracing for ARM
Message-ID: <20081121180929.GA13863@blackbean.org>
References: <20081118231525.GA16081@blackbean.org> <alpine.DEB.1.10.0811201734050.14652@gandalf.stny.rr.com> <20081120224903.GA3244@blackbean.org> <alpine.DEB.1.10.0811202114560.4464@gandalf.stny.rr.com> <20081121035509.GA5266@blackbean.org> <alpine.DEB.1.10.0811210917380.6447@gandalf.stny.rr.com> <20081121153827.GA8433@flint.arm.linux.org.uk>
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On Fri, Nov 21, 2008 at 03:38:27PM +0000, Russell King - ARM Linux wrote:
> 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?

> 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.)

The only instruction that matters for arm is "bl <func>" since that's
what's emitted by gcc to call mcount().  I suspect thumb will be easy
to support.  <func> isn't known when the file is compiled, so I assume
in that case the assembler will have to leave at least 4 bytes (even
in thumb) in case mcount() gets linked far away.

I haven't looked at the return tracing code yet.  That might be harder
to support, but given x86 works, I suspect it'll be doable.

-Jim
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