MMIO and gcc re-ordering issue

Jes Sorensen jes at
Fri May 30 19:36:20 EST 2008

James Bottomley wrote:
>> The only way to guarantee ordering in the above setup, is to either
>> make writel() fully ordered or adding the mmiowb()'s inbetween the two
>> writel's. On Altix you have to go and read from the PCI brige to
>> ensure all writes to it have been flushed, which is also what mmiowb()
>> is doing. If writel() was to guarantee this ordering, it would make
>> every writel() call extremely expensive :-(
> So if a read from the bridge achieves the same effect, can't we just put
> one after the writes within the spinlock (an unrelaxed one).  That way
> this whole sequence will look like a well understood PCI posting flush
> rather than have to muck around with little understood (at least by most
> driver writers) io barriers?


I think mmiowb() does some sort of status read from the bridge, I am not
sure if it's enough to just do a regular readl().

I'm adding Jeremy to the list, he should know for sure.


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