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    SubjectRe: cpuidle future and improvements
    On 06/18/2012 02:53 PM, Peter De Schrijver wrote:
    > On Mon, Jun 18, 2012 at 02:35:42PM +0200, Daniel Lezcano wrote:
    >> On 06/18/2012 01:54 PM, Deepthi Dharwar wrote:
    >>> On 06/18/2012 02:10 PM, Daniel Lezcano wrote:
    >>>
    >>>>
    >>>> Dear all,
    >>>>
    >>>> A few weeks ago, Peter De Schrijver proposed a patch [1] to allow per
    >>>> cpu latencies. We had a discussion about this patchset because it
    >>>> reverse the modifications Deepthi did some months ago [2] and we may
    >>>> want to provide a different implementation.
    >>>>
    >>>> The Linaro Connect [3] event bring us the opportunity to meet people
    >>>> involved in the power management and the cpuidle area for different SoC.
    >>>>
    >>>> With the Tegra3 and big.LITTLE architecture, making per cpu latencies
    >>>> for cpuidle is vital.
    >>>>
    >>>> Also, the SoC vendors would like to have the ability to tune their cpu
    >>>> latencies through the device tree.
    >>>>
    >>>> We agreed in the following steps:
    >>>>
    >>>> 1. factor out / cleanup the cpuidle code as much as possible
    >>>> 2. better sharing of code amongst SoC idle drivers by moving common bits
    >>>> to core code
    >>>> 3. make the cpuidle_state structure contain only data
    >>>> 4. add a API to register latencies per cpu
    >>>
    >>> On huge systems especially servers, doing a cpuidle registration on a
    >>> per-cpu basis creates a big overhead.
    >>> So global registration was introduced in the first place.
    >>>
    >>> Why not have it as a configurable option or so ?
    >>> Architectures having uniform cpuidle state parameters can continue to
    >>> use global registration, else have an api to register latencies per cpu
    >>> as proposed. We can definitely work to see the best way to implement it.
    >>
    >> Absolutely, this is one reason I think adding a function:
    >>
    >> cpuidle_register_latencies(int cpu, struct cpuidle_latencies);
    >>
    >> makes sense if it is used only for cpus with different latencies.
    >> The other architecture will be kept untouched.
    >>
    >> IMHO, before adding more functionalities to cpuidle, we should cleanup
    >> and consolidate the code. For example, there is a dependency between
    >> acpi_idle and intel_idle which can be resolved with the notifiers, or
    >> there is intel specific code in cpuidle.c and cpuidle.h, cpu_relax is
    >> also introduced to cpuidle which is related to x86 not the cpuidle core,
    >> etc ...
    >>
    >> Cleanup the code will help to move the different bits from the arch
    >> specific code to the core code and reduce the impact of the core's
    >> modifications. That should let a common pattern to emerge and will
    >> facilitate the modifications in the future (per cpu latencies is one of
    >> them).
    >>
    >> That will be a lot of changes and this is why I proposed to put in place
    >> a cpuidle-next tree in order to consolidate all the cpuidle
    >> modifications people is willing to see upstream and provide better testing.
    >
    > Sounds like a good idea. Do you have something like that already?

    Yes but I need to cleanup the tree before.

    http://git.linaro.org/gitweb?p=people/dlezcano/linux-next.git;a=summary

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