linux-kernel - Re: + unaligned-introduce-common-header.patch added to -mm tree

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Date:	Wed, 19 Nov 2008 09:16:26 -0800
From:	Harvey Harrison <harvey.harrison@...il.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	mm-commits@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	bryan.wu@...log.com, "David S. Miller" <davem@...emloft.net>,
	dhowells@...hat.com, Greg Ungerer <gerg@...inux.org>,
	grundler@...isc-linux.org, heiko.carstens@...ibm.com,
	hskinnemoen@...el.com, ink@...assic.park.msu.ru, kyle@...artin.ca,
	lethal@...ux-sh.org, linux-arch@...r.kernel.org, mingo@...e.hu,
	Paul Mackerras <paulus@...ba.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Russell King <rmk@....linux.org.uk>, rth@...ddle.net,
	schwidefsky@...ibm.com, takata@...ux-m32r.org, tglx@...utronix.de,
	tony.luck@...el.com, ysato@...rs.sourceforge.jp,
	zankel@...silica.com, Roman Zippel <zippel@...ux-m68k.org>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>
Subject: Re: + unaligned-introduce-common-header.patch added to -mm tree

On Wed, 2008-11-19 at 09:21 +0100, Geert Uytterhoeven wrote:
> On Tue, 18 Nov 2008, akpm@...ux-foundation.org wrote:
> > Subject: unaligned: introduce common header
> > From: Harvey Harrison <harvey.harrison@...il.com>
> > 
> > There are two common cases in the kernel, one where unaligned access is OK
> > for an arch and one where the arch uses a packed-struct for the native
> > endianness and opencoded C byteshifting for the other endianness. 
> > Consolidate these two implementations in asm-generic/unaligned.h
> > 
> > Arches that require no special handling of unaligned access can define
> > _UNALIGNED_ACCESS_OK in their asm/unaligned.h before including the generic
> > version.
> > 
> > +static inline void __put_le32_noalign(u8 *p, u32 val)
> > +{
> > +	__put_le16_noalign(p + 2, val >> 16);
> > +	__put_le16_noalign(p, val);
> 
> Isn't it more logical to reverse the order, to store in increasing memory
> locations:
> 
> 	__put_le16_noalign(p, val);
> 	__put_le16_noalign(p + 2, val >> 16);
> 

All of the byteshifting versions were cribbed from the ARM implementation.

I'm not sure if there was a particular reason for doing it in this order, but
a lot of work seems to have gone in to minimize register usage.

See include/asm-arm/unaligned.h circa 2.6.25.

Harvey

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