linux-kernel - Re: [PATCH] Add "is_power_of

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Message-ID: <Pine.LNX.4.64.0701300938580.24147@CPE00045a9c397f-CM001225dbafb6>
Date:	Tue, 30 Jan 2007 09:45:14 -0500 (EST)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Paul Mackerras <paulus@...ba.org>, dhowells@...hat.com,
	galak@...nel.crashing.org
Subject: Re: [PATCH] Add "is_power_of_2" checking to log2.h.

On Tue, 30 Jan 2007, Jan Engelhardt wrote:

>
> >> Why the qualifier? Zero *is* not a power of 2, is it?
>
> No, it is not:
>
> 	In[1]:= Solve[2^n == 0, n]
>
> 	Out[1]= {}
>
> So says Mathematica5.

oooookay, that's kind of like taking a sandblaster to a soup cracker.

seriously, though, there is the potential of breaking something with
this change since you can see how there is some inconsistency in how
it's done *now* just for powerpc which, in some places, defines its
own versions of this:

./arch/ppc/mm/pgtable.c:
	#define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))
./arch/ppc/syslib/ppc85xx_rio.c:
	#define is_power_of_2(x) (((x) & ((x) - 1)) == 0)
./arch/powerpc/mm/pgtable_32.c:
	#define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))

note how the first and third macros *won't* consider zero a power of
two, while the second one *will*.  hence the need for a single
standard for all of this, just to play it safe.

rday

p.s.  in case you missed it, that was a not-so-subtle plea to please
just apply this patch so we can move on.

-- 
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

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