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

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Message-ID: <C2D7FE5348E1B147BCA15975FBA23075F4E8F5FE@us01wembx1.internal.synopsys.com>
Date:	Fri, 11 Mar 2016 05:18:32 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Arnd Bergmann <arnd@...db.de>
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 01:15 PM, Arnd Bergmann wrote:
> 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?

OK I got the response from hardware guys that we follow the normal mode of
swapping byte order for big-endian mode. Arnd can u please explain how that might
impact the io accessors, it at all.

And what exactly are semantics of readX() and ioreadX() - even if arch specific
and I'd be glad to change that for ARC.

I can also help with documenting them properly some where as went digging into
mailing list first thing Lada posted this patch :-)

Thx,
-Vineet