linux-kernel - Re: [PATCH] drivers/block/xsysace - replace in(out)_8/in(out)_be16/in(out)

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Date:	Tue, 12 Feb 2013 11:26:10 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Michal Simek <monstr@...str.eu>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	dahinds@...rs.sourceforge.net
Subject: Re: [PATCH] drivers/block/xsysace - replace in(out)_8/in(out)_be16/in(out)_le16 with generic iowrite(read)8/16(be)

On Tuesday 12 February 2013, Michal Simek wrote:
> > In particular, ARM can run both big- and little-endian even though
> > big-endian is rarely used, so you need to know the endianess for
> > the device you are talking to rather than assume that it knows
> > what the CPU does at the time.
> 
> For high performance IPs using accessors functions is still problematic
> because there will be performance regression it means that
> from my point of view there still should be any option to "setup"
> proper endians for the driver and it can't be setup at run-time.

I did not mean you have to use a run-time detection here, although
that is often the easiest solution. If you know the endianess of
a device for a specific architecture or platform, it is totally
fine to pick that endianess at compile-time and use e.g. the
readl_relaxed() accessors on ARM to give you the lowest access
latency.

	Arnd
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