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

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

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