Re: MMIO and gcc re-ordering issue

[Jes Sorensen]

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?

Hmmm,

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.

Cheers,
Jes

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