[PATCH 7/7] ARM: tegra30: cpuidle: add LP2 driver for CPU0

[Colin Cross]

On Thu, Oct 11, 2012 at 9:37 AM, Stephen Warren <swarren at wwwdotorg.org> wrote:
> On 10/11/2012 05:24 AM, Joseph Lo wrote:
>> On Wed, 2012-10-10 at 06:49 +0800, Stephen Warren wrote:
>>> On 10/08/2012 04:26 AM, Joseph Lo wrote:
>>>> The cpuidle LP2 is a power gating idle mode. It support power gating
>>>> vdd_cpu rail after all cpu cores in LP2. For Tegra30, the CPU0 must
>>>> be last one to go into LP2. We need to take care and make sure whole
>>>> secondary CPUs were in LP2 by checking CPU and power gate status.
>>>> After that, the CPU0 can go into LP2 safely. Then power gating the
>>>> CPU rail.

<snip>

>>>> @@ -85,16 +108,22 @@ static int __cpuinit tegra30_idle_lp2(struct cpuidle_device *dev,
>>>>                                   int index)
>>>>  {
>>>>     bool entered_lp2 = false;
>>>> +   bool last_cpu;
>>>>
>>>>     local_fiq_disable();
>>>>
>>>> +   last_cpu = tegra_set_cpu_in_lp2(dev->cpu);
>>>> +   if (dev->cpu == 0) {
>>>> +           if (last_cpu)
>>>> +                   entered_lp2 = tegra30_idle_enter_lp2_cpu_0(dev, drv,
>>>> +                                                              index);
>>>> +           else
>>>> +                   cpu_do_idle();
>>>> +   } else {
>>>>             entered_lp2 = tegra30_idle_enter_lp2_cpu_n(dev, drv, index);
>>>> +   }
>>>
>>> Hmm. That means that if the last CPU to enter LP2 is e.g. CPU1, then
>>> even though all CPUs are now in LP2, the complex as a whole doesn't
>>> enter LP2. Is there a way to make the cluster as a whole enter LP2 in
>>> this case? Isn't that what coupled cpuidle is for?
>>
>> It may look like the coupled cpuidle can satisfy the usage here. But it
>> didn't. Please check the criteria of coupled cpuidle.
>
> What about the first part of the question. What happens if:
>
> CPU3 enters LP2
> CPU2 enters LP2
> CPU0 enters LP2
> CPU1 enters LP2
>
> Since CPU1 is not CPU0, tegra30_idle_enter_lp2_cpu_n() is called, and
> hence I think the whole CPU complex is never rail-gated (just each CPU
> is power-gated) even though all CPUs are in LP2 and the complex could be
> rail-gated. Isn't this missing out on power-savings?
>
> So, we either need to:
>
> a) Make tegra30_idle_enter_lp2_cpu_n() rail-gate if the last CPU is
> entering LP2, and then I'm not sure the implementation would be any
> different to tegra30_idle_enter_lp2_cpu_0, would it?
>
> b) If CPUn can't trigger rail-gating, then when CPUn is the last to
> enter LP2 of the whole complex, it needs to IPI to CPU0 to tell it to
> rail-gate, and simply power-gate itself. I believe this IPI interaction
> is exactly what coupled cpuidle is about, isn't it?
>
>> /*
>>  * To use coupled cpuidle states, a cpuidle driver must:
>>  *
>>  *    Set struct cpuidle_device.coupled_cpus to the mask of all
>>  *    coupled cpus, usually the same as cpu_possible_mask if all cpus
>>  *    are part of the same cluster.  The coupled_cpus mask must be
>>  *    set in the struct cpuidle_device for each cpu.
>>  *
>>  *    Set struct cpuidle_device.safe_state to a state that is not a
>>  *    coupled state.  This is usually WFI.
>>  *
>>  *    Set CPUIDLE_FLAG_COUPLED in struct cpuidle_state.flags for each
>>  *    state that affects multiple cpus.
>>  *
>>  *    Provide a struct cpuidle_state.enter function for each state
>>  *    that affects multiple cpus.  This function is guaranteed to be
>>  *    called on all cpus at approximately the same time.  The driver
>>  *    should ensure that the cpus all abort together if any cpu tries
>>  *    to abort once the function is called.  The function should return
>>  *    with interrupts still disabled.
>>  */
>>
>> The Tegra30 can support the secondary CPUs go into LP2 (power-gate)
>> independently.
>
> I think that just means that the safe state for CPUn (i.e. not CPU0) can
> do better than WFI on Tegra30, even though it can't on Tegra20.

Exactly.

>> The limitation of the CPU0 is the CPU0 must be the last
>> one to go into LP2 to shut off CPU rail.
>>
>> It also no need for every CPU to leave LP2 at the same time. The CPU0
>> would be always the first one that woken up from LP2. But all the other
>> secondary CPUs can still keep in LP2. One of the secondary CPUs can also
>> be woken up alone, if the CPU0 already up.
>
> That seems like an implementation detail. Perhaps coupled cpuidle needs
> to be enhanced to best support Tegra30?

As is, coupled cpuidle will work on Tegra30, but it will unnecessarily
wake up the secondary cpus during the transitions to off and back on
again.  Those cpus will immediately go back to single-cpu LP2, so it
may not be a big deal, but there is a small optimization I've
discussed with a few other people that could avoid waking them up.  I
suggest adding an extra pre-idle hook to the Tegra30 that is called by
coupled cpuidle on the last cpu to go down.  It would return a cpumask
of cpus that have been prepared for idle by guaranteeing that they
will not wake up from an interrupt, and therefore don't need to be
woken up for the transitions.  I haven't worked with a cpu that needs
this optimization yet, so I haven't done it.