linux-kernel - Re: MMIO and gcc re-ordering issue

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Message-ID: <20080604124714.55dbce65@core>
Date:	Wed, 4 Jun 2008 12:47:14 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Trent Piepho <tpiepho@...escale.com>,
	Russell King <rmk+lkml@....linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>, linux-arch@...r.kernel.org,
	scottwood@...escale.com, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org
Subject: Re: MMIO and gcc re-ordering issue

> Anyway, Intel certainly seems to document that WC memory is serialized by 
> any access to UC memory.

I don't believe that is actually true on Pentium Pro at least.

>   So what started out as a "we can do accesses to the frame buffer more 
>   efficiently without anybody ever even having to know or care" has 
>   turned into a whole nightmare of people using it for other things, and 
>   then you very much _do_ have to care! ]

Notably graphics where 3Dfx lined the registers up specifically to make
this work. We were also using it with I²O where it gave a small
performance gain.

Alan
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