linux-kernel - Re: Opinion on ordering of writel vs. stores to RAM

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Date:	Tue, 12 Sep 2006 15:30:03 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Albert Cahalan <acahalan@...il.com>
Cc:	jbarnes@...tuousgeek.org, alan@...rguk.ukuu.org.uk,
	davem@...emloft.net, jeff@...zik.org, paulus@...ba.org,
	torvalds@...l.org, linux-kernel@...r.kernel.org, akpm@...l.org,
	segher@...nel.crashing.org
Subject: Re: Opinion on ordering of writel vs. stores to RAM


> Oh no, it's great for regular device driver work. I used this
> type of system all the time on a different PowerPC OS.
> 
> Suppose you need to set up a piece of hardware. Assume that the
> hardware isn't across some nasty bridge. You do this:
> 
> hw->x = 42;
> hw->y = 19;
> eieio();
> hw->p = 11;
> hw->q = 233;
> hw->r = 87;
> eieio()
> hw->n = 101;
> hw->m = 5;
> eieio()
> 
> In that ficticious example, I get 7 writes to the hardware device
> with only 3 "eieio" operations. It's not hard at all. Sometimes
> a "sync" is used instead, also explicitly.

You can do that with my proposed __writel which is a simple store as
writes to non-cacheable and guarded storage have to stay in order
according to the PowerPC architecture. No need for __raw.

> To get even more speed, you can mark memory as non-coherent.

Ugh ? MMIO space is always marked non-coherent. You are not supposed to
set the M bit if the I is set in the page tables. If you are talking
about main memory, then it's a completely different discussion.

> You can even do this for RAM. There are cache control instructions
> to take care of any problems; simply ask the CPU to write things
> out as needed.

Sure, though that's not the topic.

> Linux should probably do this:
> 
> Plain stuff is like x86. If you want the performance of loose
> ordering, ask for it when you get the mapping and use read/write
> functions that have a "_" prefix. If you mix the "_" versions
> with a plain x86-like mapping or the other way, the behavior you
> get will be an arch-specific middle ground.

No. I want precisely defined semantics in all cases.

Ben.


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