netdev - Re: [PATCH 2/3] x86_64: Define 128-bit memory-mapped I/O operations

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Message-ID: <50352265.3050203@zytor.com>
Date:	Wed, 22 Aug 2012 11:18:13 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Laight <David.Laight@...lab.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	David Miller <davem@...emloft.net>, tglx@...utronix.de,
	mingo@...hat.com, netdev@...r.kernel.org,
	linux-net-drivers@...arflare.com, x86@...nel.org
Subject: Re: [PATCH 2/3] x86_64: Define 128-bit memory-mapped I/O operations

On 08/22/2012 11:11 AM, Ben Hutchings wrote:
> 
> Well the whole point of having the two 32-bit generic implementations is
> that hardware may care about the order!  How can it be right that a
> 64-bit implementation assumes it doesn't?
> 

On x86 platforms it is pretty much universal (as in: I have never seen
an exception) that transactions that are broken up are broken up in
littleendian order.  That was the original readq/writeq implementation
on x86-32, but it screwed up some other architectures.

The other reason to not have readq/writeq by default where we *know* it
can't be supported is that some drivers (e.g. the CNIC/OPA-2 driver I
mentioned) can do better, performance-wise, if it knows that.

	-hpa
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