linux-kernel - Re: [PATCH] NVMe: Fix compilation on architecturs without readq/writeq

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Message-ID: <CAMO-S2joH0fwEyOOCSFGM+Lc=xgiarNxm3Rj9DgWJNeoRD+QRQ@mail.gmail.com>
Date:	Sun, 5 Feb 2012 00:39:00 +0900
From:	Hitoshi Mitake <h.mitake@...il.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	James Bottomley <jbottomley@...allels.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Roland Dreier <roland@...estorage.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] NVMe: Fix compilation on architecturs without readq/writeq

On Fri, Feb 3, 2012 at 00:05, H. Peter Anvin <hpa@...ux.intel.com> wrote:
> On 02/01/2012 05:05 PM, James Bottomley wrote:
>>
>>
>> Incidentally, the last time this came up was with mpt fusion: for a
>> write to a 64 bit register, it didn't care about order, but it did care
>> about interleaving as in if you write one half of a 64 bit register and
>> then write to another register, the 64 bit register effectively gets
>> written with zeros in the part you didn't write to, so we had to put a
>> spin lock in the open coded writeb/w/l/q() to make sure the card didn't
>> get interleaved writes.
>>
>
> There are always going to be hardware which have specific needs, and for
> those open-coding makes sense, but the littleendian/bigendian pair is going
> to cover ~90% of users and make sense to can.
>
> I worked myself on a driver (which sadly never shipped) which had an WC
> window and a UC window... the final write in a series had a completion bit
> in it and would go to the UC window after setting up a whole chunk of
> operations in the WC window (writing UC memory flushes WC memory ahead of
> it.)
>
> Thus, the two-part breakdown of writeq() to the UC window had to write the
> low half to the WC window instead.  This is clearly not generic.
>
>        -hpa
>
>

Because of my ignorant, I don't know the words "UC window" and "WC window"
in this context. Could you teach me?

-- 
Hitoshi Mitake
h.mitake@...il.com
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