MMIO and gcc re-ordering issue

[Trent Piepho]

On Tue, 3 Jun 2008, Linus Torvalds wrote:
> On Tue, 3 Jun 2008, Nick Piggin wrote:
>>
>> Linus: on x86, memory operations to wc and wc+ memory are not ordered
>> with one another, or operations to other memory types (ie. load/load
>> and store/store reordering is allowed). Also, as you know, store/load
>> reordering is explicitly allowed as well, which covers all memory
>> types. So perhaps it is not quite true to say readl/writel is strongly
>> ordered by default even on x86. You would have to put in some
>> mfence instructions in them to make it so.

So on x86, these could be re-ordered?

writel(START_OPERATION, CONTROL_REGISTER);
status = readl(STATUS_REGISTER);

> Well, you have to ask for WC/WC+ anyway, so it's immaterial. A driver that
> does that needs to be aware of it. IOW, it's a non-issue, imnsho.

You need to ask for coherent DMA memory too.