From: k.kozlowski@samsung.com (Krzysztof Kozlowski)
Date: Thu, 11 Jun 2015 09:35:44 +0900
Subject: [PATCH RFC 3/3] PM / Domains: Introduce generic PM domain for cpu
 domain
In-Reply-To: <7hbngnpkfd.fsf@deeprootsystems.com>
References: <1433456946-53296-1-git-send-email-lina.iyer@linaro.org>
 <1433456946-53296-4-git-send-email-lina.iyer@linaro.org>
 <5574121F.2000806@samsung.com> <7hbngnpkfd.fsf@deeprootsystems.com>
Message-ID: <5578D7E0.8020305@samsung.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On 11.06.2015 02:01, Kevin Hilman wrote:
> Krzysztof Kozlowski <k.kozlowski@samsung.com> writes:
> 
>> W dniu 05.06.2015 o 07:29, Lina Iyer pisze:
>>> Generally cpus are grouped under a power domain in a SoC. When all cpus
>>> in the domain are in their power off state,
>>
>> What do you exactly mean here by "CPU in power off state"? How does it
>> map to kernel understanding of CPU device (hotplug? cpuidle?)?
>>
>>> the cpu domain can also be
>>> powered off. Genpd provides the framework for defining cpus as devices
>>> that are part of a cpu domain.
>>
>> The problem which is solved looks to me like the same problem which
>> coupled cpuidle tried to solve: a certain deep sleep mode (e.g. power
>> off) can be entered when whole cluster is idle or other CPUs in cluster
>> are powered off completely.
>>
>> It seems a little like duplicating the effort around coupled cpuidle.
> 
> Yes, it duplicates some aspects of coupled idle states, but coupled
> states have their own limitations:
> 
> - only handles CPUs, not other devices sharing a power rail (e.g. L2$,
>   GIC, floating point unit, CoreSight, etc. etc.)
> 
> - not scaling well past 2 CPUs
> 
> - doesn't handle clusters: While this series only addresses CPUs
>   currently, the approach can be extended.  Because genpd handles nested
>   domains, the could be used to model clusters as well.

Right. I agree with your explanation. I am just thinking how to utilize
this for Exynos deep sleep modes which now we implement using coupled
cpuidle.

Anyway I like the idea!

Best regards,
Krzysztof