Re: [PATCH] Document Linux's memory barriers

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Tue, 7 Mar 2006, Alan Cox wrote:
[SNIPPED...]
>
> Not always. MMIO ordering is outside of the CPU ordering rules and into
> PCI and other bus ordering rules. Consider
>
> 	writel(STOP_DMA, &foodev->ctrl);
> 	free_dma_buffers(foodev);
>
> This leads to horrible disasters.

This might be a good place to document:
    dummy = readl(&foodev->ctrl);

Will flush all pending writes to the PCI bus and that:
    (void) readl(&foodev->ctrl);
... won't because `gcc` may optimize it away. In fact, variable
"dummy" should be global or `gcc` may make it go away as well.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.15.4 on an i686 machine (5589.50 BogoMips).
Warning : 98.36% of all statistics are fiction, book release in April.
_


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