Re: [PATCH] arm: dts: exynos5: Remove multi core timer

[Tomasz Figa <tomasz.figa@gmail.com>]

On 15.05.2014 23:44, Kukjin Kim wrote:
> On 05/16/14 06:33, Doug Anderson wrote:
>> Tomasz,
>>
>> On Thu, May 15, 2014 at 2:14 PM, Tomasz Figa<tomasz.figa@gmail.com> 
>> wrote:
>>> Hi Chirantan,
>>>
>>> On 15.05.2014 23:07, Chirantan Ekbote wrote:
>>>> The multi core timer and the ARM architected timer are two different
>>>> interfaces to the same underlying hardware timer.  This causes some
>>>> strange timing issues when they are both enabled at the same time so
>>>> remove the mct from the device tree and keep only the architected
>>>> timer.
>>>
>>> Huh? I've always thought MCT is a completely separate hardware block
>>> outside of ARM cores, while architected timers are embedded inside the
>>> CPU block in which the ARM cores reside. Could you elaborate on this?
>>
>> Yup.  Our thoughts exactly.
>>
>> ...but it appears not to be the case.  Chirantan demonstrated this in
>> U-Boot just to prove that it's not some strange kernel interaction in
>> <https://chromium-review.googlesource.com/200035>.  I took his patch
>> and tweaked it a little more myself in
>> <https://chromium-review.googlesource.com/200098>.
>>
>> Specifically:
>> * If you stop the MCT, the arch timer stops
>> * If you reset the MCT, the arch timer resets
>> * If you start the MCT again, the arch timer starts again
>> * If you read the MCT and the arch timer, they give the same value.
>>
>>
>> This is apparently the answer to my question at
>> <http://www.spinics.net/lists/linux-samsung-soc/msg29085.html>.
>> Specifically Chirantan found that the big jump in time happened when
>> MCT reset to 0.  That made the arch timer code think that there was a
>> wraparound and jump forward in time a lot.
>>
>>
>> Please confirm if you have a system that has MCT and arch timer in
>> front of you.
>>
> Hi all,
> 
> I need to talk to hardware guy to clarify the issue then I'll let you know.

Great, thanks. Having a confirmation from hardware team would be
definitely nice.

Best regards,
Tomasz