linux-kernel - Re: [PATCH] asm-generic: architecture independent readq/writeq for 32bit environment

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Date:	Wed, 8 Feb 2012 08:28:18 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Hitoshi Mitake <h.mitake@...il.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Roland Dreier <roland@...estorage.com>,
	Matthew Garrett <mjg@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Bottomley <James.Bottomley@...allels.com>,
	Ravi Anand <ravi.anand@...gic.com>,
	James Bottomley <jbottomley@...allels.com>,
	Len Brown <lenb@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Uhlenkott <juhlenko@...mai.com>,
	Kashyap Desai <Kashyap.Desai@....com>,
	linux-kernel@...r.kernel.org,
	Vikas Chaudhary <vikas.chaudhary@...gic.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>, mingo@...e.hu
Subject: Re: [PATCH] asm-generic: architecture independent readq/writeq for
 32bit environment

On Wed, Feb 8, 2012 at 8:14 AM, Hitoshi Mitake <h.mitake@...il.com> wrote:
> On Tue, Feb 7, 2012 at 12:56, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> On Feb 6, 2012 6:47 PM, "Hitoshi Mitake" <h.mitake@...il.com> wrote:
>>>
>>>  All of them are endian aware
>>
>> I think that part is wrong.
>>
>> Pci is little-endian, so readl and writel are always already little-endian.
>> Trying to make readq be endian-aware is wrong.
>
> Is every memory area which can be read/written by read[wl]/write[wl]
> always little-endian?
>
> If so, of course I have to eliminate the big-endian version.
> But I'm not sure about this point because the new readq/writeq is
> in asm-generic which is used by every archs, so I'd like to ask you about it.

Yes, PCI is always little-endian everywhere.

Now, some architectures may have some unified IO space where parts of
it is PCI, and other parts are programmed the same way but is for some
other bus, but the PCI part will always be little-endian because
otherwise no common driver would ever work. But those non-PCI things
would never be relevant for an emulated readq/writeq anyway, and would
need some bus-specific accessor functions.

                 Linus
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