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

[Johannes Berg <johannes@sipsolutions.net>]

On Mon, 2015-02-02 at 11:36 -0800, Peter Oh wrote:
> On 02/02/2015 11:22 AM, Johannes Berg wrote:
> >>> 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.
> >> since it's mmio, iowrite will be map to write, not out which is cached
> >> mapping.
> >> That's why we address "posted write" here.
> >> If it's un-cached mapping which is volatile, we don't even need ioread.
> > No, this isn't true - "posted write" in the context of this discussion
> > is about the PCIe bus. Memory writes that go through cache aren't
> > referred to as "posted writes", those are just (cached) memory writes.
> >
> >>>    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.
> >> So how does ioread guarantee PCIe bus transaction done?
> > That's how PCIe works, operations are serialized, and read() has to wait
> > for a response from the device
> do you know which mechanism or which instruction set makes read() wait 
> for a response from the device?

I have no idea. I assume it's just like a DRAM read, the CPU stalls
while there's no response.

johannes


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