Subject: Re: Opinion on ordering of writel vs. stores to RAM
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Segher Boessenkool <segher@kernel.crashing.org>,
       Jesse Barnes <jbarnes@virtuousgeek.org>,
       David Miller <davem@davemloft.net>, jeff@garzik.org, paulus@samba.org,
       torvalds@osdl.org, linux-kernel@vger.kernel.org, akpm@osdl.org
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Date: Mon, 11 Sep 2006 10:02:52 +0100
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Ar Llu, 2006-09-11 am 10:12 +1000, ysgrifennodd Benjamin Herrenschmidt:
>  - writel/readl are fully synchronous (minus mmiowb for spinlocks)
>  - we provide __writel/__readl with some barriers with relaxed ordering
> between memory and MMIO (though still _precise_ semantics, not arch
> specific)

I'd rather they were less precise than your later proposal but that
reflects the uses I'm considering perhaps.

>  * Option B:
> 
>  - The driver decides at ioremap time wether it wants a fully ordered
> mapping or not using

That is expensive because writel/readl end up full of if() while at the
moment they are often 1 instruction.


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