Re: [PATCH RFC v1 7/8] drivers: qcom: cpu_pd: Handle cpu hotplug in the domain

[Lina Iyer <ilina@codeaurora.org>]

On Fri, Oct 12 2018 at 11:01 -0600, Sudeep Holla wrote:
>On Fri, Oct 12, 2018 at 10:04:27AM -0600, Lina Iyer wrote:
>> On Fri, Oct 12 2018 at 09:04 -0600, Sudeep Holla wrote:
>
>[...]
>
>Yes all these are fine but with multiple power-domains/cluster, it's
>hard to determine the first CPU. You may be able to identify it within
>the power domain but not system wide. So this doesn't scale with large
>systems(e.g. 4 - 8 clusters with 16 CPUs).
>
We would probably not worry too much about power savings in a msec
scale, if we have that big a system. The driver is a platform specific
driver, primarily intended for a mobile class CPU and usage. In fact, we
haven't done this for QC's server class CPUs.

>> > I think we are mixing the system sleep states with CPU idle here.
>> > If it's system sleeps states, the we need to deal it in some system ops
>> > when it's the last CPU in the system and not the cluster/power domain.
>> >
>> I think the confusion for you is system sleep vs suspend. System sleep
>> here (probably more of a QC terminology), refers to powering down the
>> entire SoC for very small durations, while not actually suspended. The
>> drivers are unaware that this is happening. No hotplug happens and the
>> interrupts are not migrated during system sleep. When all the CPUs go
>> into cpuidle, the system sleep state is activated and the resource
>> requirements are lowered. The resources are brought back to their
>> previous active values before we exit cpuidle on any CPU. The drivers
>> have no idea that this happened. We have been doing this on QCOM SoCs
>> for a decade, so this is not something new for this SoC. Every QCOM SoC
>> has been doing this, albeit differently because of their architecture.
>> The newer ones do most of these transitions in hardware as opposed to an
>> remote CPU. But this is the first time, we are upstreaming this :)
>>
>
>Indeed, I know mobile platforms do such optimisations and I agree it may
>save power. As I mentioned above it doesn't scale well with large systems
>and also even with single power domains having multiple idle states where
>only one state can do this system level idle but not all. As I mentioned
>in the other email to Ulf, it's had to generalise this even with DT.
>So it's better to have this dealt transparently in the firmware.
>
Good, then we are on agreement here.
But this is how this platform is. It cannot be done in firmware and what
we doing here is a Linux platform driver that cleans up nicely without
having to piggy back on an external dependency.

Thanks,
Lina