Re: [PATCH] ath10k: Replace ioread with wmb for data sync

[Johannes Berg <johannes@sipsolutions.net>]

On Mon, 2015-02-02 at 09:33 -0800, Peter Oh wrote:

> > The code (as it is before your patch) implies that it's trying to make
> > sure that before it continues, any previous writes to the PCIe device's
> > registers are posted. The only way to ensure that is to do a read to the
> > registers, as the code does now.

> Do you know how the read ensure that although the read code does not 
> check the return value?
> Can you explain how a read ensures that posted write reaches PCIe device?

You basically have the following sequence:

iowrite()
ioread()

If you look, you'll see that iowrite() is actually done (or should be,
or perhaps with appropriate syncs) on an uncached mapping. As a result,
the only thing you care about here is the PCIe bus, not the CPU cache
flush. And from there on that's just a question of PCIe bus semantics.

johannes