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

[Tomasz Figa]

On 15.05.2014 23:54, Doug Anderson wrote:
> Tomasz,
> 
> 
> 
> On Thu, May 15, 2014 at 2:40 PM, Tomasz Figa <tomasz.figa at gmail.com> wrote:
>> On 15.05.2014 23:33, Doug Anderson wrote:
>>> Tomasz,
>>>
>>> On Thu, May 15, 2014 at 2:14 PM, Tomasz Figa <tomasz.figa at 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.
>>
>> Wow, this is so strange that I just can't believe it, but if you proved
>> it using such detailed test then I don't have any reasons to object anymore.
>>
>> One more thing, though, is whether the architected timer "interface" can
>> wake the system from lighter power states, such as AFTR or LPA. Could
>> you check this?
> 
> I've never used AFTR or LPA and so can't really comment on it.
> Hopefully someone on the list can?
> 
> NOTE: if for some reason we need to keep the MCT around, we're
> definitely going to need to account for the fact that tweaking it
> affects the arch timer.  ...and having the arch timer is really nice
> since:

[Let me reorder the points below to make it easier to comment:]

> * it's faster to access.
> * it is accessible from userspace for really fast access.

Do you have some data on whether it is a significant difference,
especially considering real use cases?

> * it implements the bits needed for udelay() to use it.

Hmm, do you know what bits are those? On Exynos4 MCT is the only option
and it would be nice to let udelay() use it.

Best regards,
Tomasz