linux-kernel - Re: [PATCH v3 1/6] arm64: Add ftrace support

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Date:	Wed, 12 Feb 2014 12:03:11 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Takahiro Akashi <takahiro.akashi@...aro.org>
Cc:	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	mingo <mingo@...hat.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"gkulkarni@...iumnetworks.com" <gkulkarni@...iumnetworks.com>,
	"Bird, Tim" <tim.bird@...ymobile.com>,
	"arndb@...db.de" <arndb@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Patch Tracking <patches@...aro.org>
Subject: Re: [PATCH v3 1/6] arm64: Add ftrace support

On Wednesday 12 February 2014, Takahiro Akashi wrote:
> > If I build a large "randconfig" kernel or "allyesconfig", the kernel image
> > size exceeds 32MB, which means I can no longer link callers with a 26
> > bit signed immediate argument like the "bl _mcount" here. For any other
> > function calls, "gcc -mlong-calls" can be used to work around this, but
> > this particular instance is created by inserting assembly statements
> > into the output without considering the long-call options. Is there
> > a way to get the kernel to link anyway?
> 
> I hope that this won't happen on arm64 because a branch offset range is
> +/- 128MB.

Yes, we should be safe for the next decade on arm64. My question was really
about arm32 here, hoping someone has an idea.

> > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> >
> > Here, again, you an remove the #ifdef by making the entire file built
> > only for CONFIG_FUNCTION_GRAPH_TRACER.
> 
> You're right as far as patch [1/6] is concerned, but once patch [2/6] is
> applied,
> this file (ftrace.c) may be compiled even if CONFIG_FUNCTION_GRAPH_TRACER
> is not enabled. (That is CONFIG_DYNAMIC_FTRACE.)
> Sorry for any confusion by poorly-divided patchset.

No, that was my fault. I saw it later after I had already sent out the
message.

	Arnd
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