[RFC] ARM Generic Timer + Interaction with WFI on Cortex-A15

[Marc C]

Hi Lorenzo,

> So what's the problem then ? Just avoid adding CPUIDLE_FLAG_TIMER_STOP
> to the C-state flags and you are all set, or I am missing something
> here ?

The C3STOP flag prevents the kernel from using the timer as a
high-resolution clock source. There are some patches that remove the
C3STOP flag [1] in order for the timer to function for use with hrtimer.
I think something similar could be manageable as a DT property on the
armv7-timer binding.

> Yes I do object. Timer binding is global in the DT and do not want to
> override the flag for all local timers when, as I mentioned, A15
> behaviour is just an exception. If you really need that, please write
> an idle driver that does not enter broadcast mode on C-state entry
> (see above).

So what you're saying is that you'll outright disapprove of any patch
that would otherwise help ensure the kernel would run on any/all
variations of armv7-timer? I would imagine that we'd want things to be
more inclusive, and since there are quite a few SoCs with the timer that
behave in this manner.

I'm not trying to be a thorn in your side. I just want to make sure
everyone that has an implementation similar to ours is covered, too.

I appreciate your feedback.

Thanks,
Marc

[1] https://lkml.org/lkml/2013/6/16/245

[1] https://lkml.org/lkml/2013/6/16/245
On 11/26/13, 2:10 AM, Lorenzo Pieralisi wrote:
> Hi Marc,
> 
> On Tue, Nov 26, 2013 at 06:42:03AM +0000, Marc C wrote:
>> Hello Lorenzo,
>>
>>> It is an A15 oddity and we should not care, given that this behaviour
>>> is platform specific and likely to fail in most common A15
>>> implementations.
>>
>> I'm supporting a platform where this "oddity" is actually a relied-upon
>> feature. Our ARMv7-compliant MPCore implements the ARM Generic Timer per
>> spec. Our implementation isn't a constituent of a big.LITTLE
>> arrangement, and we'll never completely power-off all cores (we just use
>> WFI).
> 
> So what's the problem then ? Just avoid adding CPUIDLE_FLAG_TIMER_STOP
> to the C-state flags and you are all set, or I am missing something here ?
> 
>> According to the documentation that currently exists, it seems that the
>> behavior of the ARM Generic Timer on cores like the A15 is really just
>> an attribute of that specific implementation. As you've alluded to,
>> there may be other implementations that are also usable when the CPUs
>> enter WFI. That said, do you object to having an optional boolean
>> property in the arch_timer DT binding [1] which allows users to override
>> the C3STOP flag? The default behavior would be as is currently
>> implemented, and for the odd machines we can add the new property to the DT.
> 
> Yes I do object. Timer binding is global in the DT and do not want to
> override the flag for all local timers when, as I mentioned, A15 behaviour is
> just an exception. If you really need that, please write an idle driver that
> does not enter broadcast mode on C-state entry (see above).
> 
> Thanks,
> Lorenzo
>