[PATCH] Document Linux's memory barriers

[linux-os (Dick Johnson)]

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.
_


****************************************************************
The information transmitted in this message is confidential and may be privileged.  Any review, retransmission, dissemination, or other use of this information by persons or entities other than the intended recipient is prohibited.  If you are not the intended recipient, please notify Analogic Corporation immediately - by replying to this message or by sending an email to DeliveryErrors at analogic.com - and destroy all copies of this information, including any attachments, without reading or disclosing them.

Thank you.