linux-kernel - Re: [PATCH 2/5] powerpc: ftrace, convert to new dynamic ftrace arch API

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Date:	Mon, 24 Nov 2008 13:49:36 +1100
From:	Michael Ellerman <michael@...erman.id.au>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Milton Miller <miltonm@....com>,
	linuxppc-dev@...abs.org, Steven Rostedt <srostedt@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/5] powerpc: ftrace, convert to new dynamic ftrace
 arch API

On Mon, 2008-11-24 at 12:43 +1100, Paul Mackerras wrote:
> Steven Rostedt writes:
> 
> > Thanks to Paul Mackennas for pointing out the mistakes of my original
> 
> Mackerras
> 
> > +static int test_24bit_addr(unsigned long ip, unsigned long addr)
> > +{
> > +	long diff;
> > +
> > +	/*
> > +	 * Can we get to addr from ip in 24 bits?
> > +	 *  (26 really, since we mulitply by 4 for 4 byte alignment)
> > +	 */
> > +	diff = addr - ip;
> > +
> > +	/*
> > +	 * Return true if diff is less than 1 << 25
> > +	 *  and greater than -1 << 26.
> > +	 */
> > +	return (diff < (1 << 25)) && (diff > (-1 << 26));
> 
> I think this still isn't right, and the comment is one of those ones
> that is only useful to people who can't read C, as it's just a
> transliteration of the code.
> 
> The comment should say something like "Return true if diff can be
> represented as a 26-bit twos-complement binary number" and the second
> part of the test should be (diff >= (-1 << 25)).  However, since you
> define a test_offset() function in patch 4/5 that does the same test
> but using only one comparison instead of two, why don't you just say:
> 
> 	return !test_offset(diff);
> 
> (having first moved test_offset() before test_24bit_addr)?

Or better still, split out and use the code from create_branch() in
arch/powerpc/lib/code-patching.c .. which is hopefully correct :)

cheers

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person

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