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

[Catalin Marinas]

On 31 May 2012 15:46,  <snakky.zhang at gmail.com> wrote:
> On 2012年05月31日 15:21, bill4carson wrote:
>> On 2012年05月31日 14:56, Catalin Marinas wrote:
>>> On Thu, May 31, 2012 at 06:51:22AM +0100, bill4carson wrote:
>>>> On 2012年05月31日 13:19, Catalin Marinas wrote:
>>>>> On 31 May 2012 13:06, bill4carson<bill4carson at gmail.com>   wrote:
>>>>>> On 2012年05月31日 11:58, Catalin Marinas wrote:
>>>>>>> I still didn't fully understand what the problem is. So, to make
>>>>>>> sure,
>>>>>>> if you run some applications from flash using a yaffs filesystem, you
>>>>>>> get random crashes. Is this correct? If yes, a solution is to
>>>>>>> actually
>>>>>>> call flush_dcache_page() on the CPU that does the page copying from
>>>>>>> flash into RAM, which could be the yaffs filesystem.
>>>>>>
>>>>>>
>>>>>> The story goes like this:
>>>>>> function "flush_dcache_page" should be global effective
>>>>>> but in ARMv6 MPCore, it was not, it was just local effective due
>>>>>> to hardware design.
>>>>>
>>>>>
>>>>> Yes, I know this.
>>>>
>>>>
>>>> Then, why not fix "flush_dcache_page" to make it globally effective?
>>>
>>>
>>> Performance?
>>>
>>> And it's also just ARM11MPCore microarchitecture specific.
>>>
> Yes, seems newer CPUs has no such limitation thus this function is global
> effective naturally. :-)
>
> And , I find Mips's c-r4k also has this issue but it use IPI to make it.
> Details in arch/mips/mm/c-r4k.c.

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):

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.

-- 
Catalin