Query about: ARM11 MPCore: preemption/task migration cache coherency

[snakky.zhang@gmail.com (snakky.zhang at gmail.com)]

On 2012?06?01? 11:25, Catalin Marinas wrote:
> On Fri, Jun 01, 2012 at 02:11:59AM +0100, snakky.zhang at gmail.com wrote:
>> On 2012?06?01? 00:04, Catalin Marinas wrote:
>>> Rather than IPI we would better use the read-for-ownership trick like
>>> in this patch to make flush_dcache_page global (no need for
>>> write-for-ownership):
>> I think write for ownership is necessary for flush_dcache_xxx: Read
>> guarantee local data cache get newest data, at the same time write
>> guarantee the data can be flushed into memory.
>>
>> See Section 7.1 of ARM11 MPCore. Processor Technical Reference
>> Manual(Revision: r2p0) P146/728:
>> ======
>> Clean Applies to write-back data caches. If the cache line targeted by
>> the Clean operation contains stored data that has not yet been written
>> out to main memory, it is written to main memory, and the line is
>> marked as clean.
>> ======
>>
>> So I am afraid without the write action, the "clean&  invalidate"
>> action later will not write data back to main memory.
> If there is a dirty cache line, it will be written to memory by the
> clean&invalidate operation. If the data in the cache line is in a clean
> state, it means that it is identical to the main memory (or L2 if
> present).
>
> With just a read, clean&invalidate would not invalidate (remove) the
> cache lines from the other CPUs. Doing a write forces the cache line to
> only be present on the current CPU (though automatically invalidating it
> on the other CPUs).
>
>> Another question here:  Why the flush_kern_dcache_xxx in
>> arch/arm/mm/cache-v6 use "clean&  invalidate" progress instead of
>> "clean"? Seems clean is enough here.
> I think in the context of VIPT caches clean would be enough.
>
>>> http://dchs.spinics.net/lists/arm-kernel/msg125075.html
>>>
>>> (it may no longer apply, I haven't checked it for some time).
>>>
>>> That's the first thing. Secondly you still need preemption disable so
>>> that it is not preempted between RFO and the actual cache cleaning.
>> PREEMPT. :-)
>>
>> Get it. But currently, I can't find anything related to ARMv6 MPCore
>> conflict with PREEMPT. So if it is also necessary to add something in
>> Documentation and related Kconfig to describe it and make sure PREEMPT
>> can't been enabled on such CPUs?
> Well, we either get it to work or, if not possible, we add a comment.
> Let's try the former option first :)
>
Thanks for your patient explanation. :-)

Thanks a lot!
Xiao