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

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Date:	Sat, 09 Sep 2006 16:06:36 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	torvalds@...l.org, linux-kernel@...r.kernel.org,
	benh@...nel.crashing.org, akpm@...l.org,
	segher@...nel.crashing.org, davem@...emloft.net
Subject: Re: Opinion on ordering of writel vs. stores to RAM

Ar Sad, 2006-09-09 am 12:03 +1000, ysgrifennodd Paul Mackerras:
> Currently we have a sync instruction after the store in writel() but
> not one before.  The sync after is to keep the writel inside
> spinlocked regions and to ensure that the store is ordered with
> respect to the load in readl() and friends.

The spinlock v writel case is not guaranteed on other platforms and
requires you use mmiowb. The main memory v writel ordering is half
guaranteed for PCI bus only .. viz:

The following is ok

	fooblock->command = 1;
	writel(&fooblock_phys, pci_addr);   /* mem write seen */

The following is not

	fooblock->command = 1;
	writel(&fooblock_phys, pci_addr);
	fooblock->command = 2;		/* could occur before PCI */


Alan
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