Re: [linux-pm] [PATCH 0/3] coupled cpuidle state support

[Vincent Guittot]

Hi Colin,

Sorry for this late reply

On 27 January 2012 18:32, Colin Cross <ccross@xxxxxxxxxxx> wrote:
> On Fri, Jan 27, 2012 at 12:54 AM, Vincent Guittot
> <vincent.guittot@xxxxxxxxxx> wrote:
>> On 20 January 2012 21:40, Colin Cross <ccross@xxxxxxxxxxx> wrote:
>>> On Fri, Jan 20, 2012 at 12:46 AM, Daniel Lezcano
>>> <daniel.lezcano@xxxxxxxxxx> wrote:
>>>> Hi Colin,
>>>>
>>>> this patchset could be interesting to resolve in a generic way the cpu
>>>> dependencies.
>>>> What is the status of this patchset ?
>>>
>>> I can't do much with it right now, because I don't have any devices
>>> that can do SMP idle with a v3.2 kernel.  I've started working on an
>>> updated version that avoids the spinlock, but it might be a while
>>> before I can test and post it.  I'm mostly looking for feedback on the
>>> approach taken in this patch, and whether it will be useful for other
>>> SoCs besides Tegra and OMAP4.
>>>
>>
>> Hi Colin,
>>
>> In your patch, you put in safe state (WFI for most of platform) the
>> cpus that become idle and these cpus are woken up each time a new cpu
>> of the cluster becomes idle. Then, the cluster state is chosen and the
>> cpus enter the selected C-state. On ux500, we are using another
>> behavior for synchronizing  the cpus. The cpus are prepared to enter
>> the c-state that has been chosen by the governor and the last cpu,
>> that enters idle, chooses the final cluster state (according to cpus'
>> C-state). The main advantage of this solution is that you don't need
>> to wake other cpus to enter the C-state of a cluster. This can be
>> quite worth full when tasks mainly run on one cpu. Have you also think
>> about such behavior when developing the coupled cpuidle driver ? It
>> could be interesting to add such behavior.
>
> Waking up the cpus that are in the safe state is not done just to
> choose the target state, it's done to allow the cpus to take
> themselves to the target low power state.  On ux500, are you saying
> you take the cpus directly from the safe state to a lower power state
> without ever going back to the active state?  I once implemented Tegra

yes it is

> that way, and it required lots of nasty synchronization to prevent
> resetting the cpu at the same time that it was booting due to an
> interrupt, and I was later told that Tegra can't handle that sequence
> at all, although I haven't verified it yet.

you have to 2 main things to check :
 - this cpu is the last one to enter an idle state
 - other cpus are prepared to enter a cluster power state
 - other cpus are in WFI

Vincent

>
> On platforms that can't turn the cpus off in a random order, or that
> can't take a cpu directly from the safe state to the target state,
> something like these coupled cpuidle patches are required.  On
> platforms that can, the low power modes can be implemented without
> these patches, although it is very hard to do without race conditions.
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