Re: How to write portable MMIO code?

arjan
2002-04-30T19:59:56Z

In article <Pine.LNX.4.44.0204301112520.32217-100000@chaos.physics.uiowa.edu> you wrote:
>> - should one in general (i.e., assuming the worst case) do wmb() on
>> writes, and mb() on reads?

readl() and writel() are specified to contain the proper barriers already.

> I don't think mb() will help you. You're probably experiencing PCI posting
> problems - when a writel() has executed, that doesn't necessarily mean
> that the transaction has actually happened it may (and will) be buffered
> for a potentially long time.
>
> However, PCI won't reorder reads vs. writes, so you when you want to be
> sure that a write() actually reached the hardware, do a dummy read()
> afterwards, that'll flush the write buffer.

yup