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

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Date:	Mon, 24 Nov 2008 12:43:23 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Thomas Gleixner <tglx@...utronix.de>, linuxppc-dev@...abs.org,
	Milton Miller <miltonm@....com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 2/5] powerpc: ftrace, convert to new dynamic ftrace arch API

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)?

Paul.
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