MMIO and gcc re-ordering issue

[David Miller]

From: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Date: Tue, 27 May 2008 11:33:46 +1000

> Quick summary: gcc is happily re-ordering readl/writel vs. surrounding
> memory accesses (and thus accesses to DMA coherent memory) which is
> obviously a _BAD_THING_.
> 
> This is on all archs. Quick fix is to stick a "memory" clobber in all arch
> implementations of readl/writel/... (ie, making them a barrier()).
> 
> However, I'm using that as an excuse to bring back my pet subject, which
> is basically, should we instead just finally mandate the use of explicit
> rmb/wmb/mb's (which boils down to barrier() on x86) to drivers who want
> to order memory consistent accesses vs. MMIO ?

This is basically what drivers are effectively doing.