linux-kernel - Re: [PATCH] arc: use little endian accesses

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Message-Id: <201603100845.30602.arnd@arndb.de>
Date:	Thu, 10 Mar 2016 08:45:30 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	Lada Trimasova <Lada.Trimasova@...opsys.com>,
	"linux-snps-arc@...ts.infradead.org" 
	<linux-snps-arc@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	Noam Camus <noamc@...hip.com>
Subject: Re: [PATCH] arc: use little endian accesses

On Thursday 10 March 2016, Vineet Gupta wrote:
> On Wednesday 09 March 2016 10:51 PM, Lada Trimasova wrote:
> > Memory access primitives should use cpu_to_le16, cpu_to_le32, le16_to_cpu
> > and le32_to_cpu because it is not really guaranteed that drivers handles
> > any ordering themselves.
> 
> That is the driver issue. readxx as API simply returns data in native endianness.
> We've had EZChip running big endian and so far and they didn't need this change.

Most drivers using readl() or readl_relaxed() expect those to perform byte
swaps on big-endian architectures, as the registers tend to be fixed endian,
so the change seems reasonable.

It depends a little bit on how endian mode is implemented in the CPU: do you
follow the normal model of swapping byte order in the load/store instructions
of the CPU when running big-endian, or is the CPU always running little-endian
but the bus addresses get mangled on byte accesses to give the illusion
of a big-endian system?

	Arnd