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    SubjectRe: [linux-pm] [PATCH 0/8] Suspend block api (version 8)
    On Thu, 2010-06-03 at 11:03 +0100, Alan Cox wrote:
    > > [mtg: ] This has been a pain point for the PM_QOS implementation.
    > They change the constrain back and forth at the transaction level of
    > the i2c driver. The pm_qos code really wasn't made to deal with such
    > hot path use, as each such change triggers a re-computation of what
    > the aggregate qos request is.
    > That should be trivial in the usual case because 99% of the time you can
    > hot path
    > the QoS entry changing is the latest one
    > there have been no other changes
    > If it is valid I can use the cached previous aggregate I cunningly
    > saved in the top QoS entry when I computed the new one
    > (ie most of the time from the kernel side you have a QoS stack)

    Why would the kernel change the QoS state of a task? Why not have two
    interacting QoS variables, one for the task, one for the subsystem in
    question, and the action depends on their relative value?

    > > We've had a number of attempts at fixing this, but I think the
    > proper fix is to bolt a "disable C-states > x" interface into cpu_idle
    > that bypases pm_qos altogether. Or, perhaps add a new pm_qos API that
    > does the equivalent operation, overriding whatever constraint is
    > active.
    > We need some of this anyway for deep power saving because there is
    > hardware which can't wake from soem states, which in turn means if that
    > device is active we need to be above the state in question.

    Right, and I can imagine that depending on the platform details and not
    the device details, so we get platform hooks in the drivers, or possible
    up in the generic stack because I don't think NICs actually know if
    there are open connections.

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      Last update: 2010-06-03 12:07    [W:0.020 / U:7.780 seconds]
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