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

[Peter Oh <poh@codeaurora.org>]

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?
>   (but write doesn't - which is "posted
> write")
>
> johannes
>