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    SubjectRe: [linux-pm] [PATCH 0/8] Suspend block api (version 8)
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    On Thu, 2010-06-03 at 09:58 -0700, Kevin Hilman wrote:
    > "Gross, Mark" <mark.gross@intel.com> writes:
    >
    > >>-----Original Message-----
    > >>From: Kevin Hilman [mailto:khilman@deeprootsystems.com]
    > >>Sent: Thursday, June 03, 2010 7:43 AM
    > >>To: Peter Zijlstra
    > >>Cc: Alan Cox; Gross, Mark; Florian Mickler; James Bottomley; Arve
    > >>Hjønnevåg; Neil Brown; tytso@mit.edu; LKML; Thomas Gleixner; Linux OMAP
    > >>Mailing List; Linux PM; felipe.balbi@nokia.com
    > >>Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)
    > >>
    > >>Peter Zijlstra <peterz@infradead.org> writes:
    > >>
    > >>> 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?
    > >>
    > >>Yes, having a QoS parameter per-subsystem (or even per-device) is very
    > >>important for SoCs that have independently controlled powerdomains.
    > >>If all devices/subsystems in a particular powerdomain have QoS
    > >>parameters that permit, the power state of that powerdomain can be
    > >>lowered independently from system-wide power state and power states of
    > >>other power domains.
    > >>
    > > This seems similar to that pm_qos generalization into bus drivers we where
    > > waving our hands at during the collab summit in April? We never did get
    > > into meaningful detail at that time.
    >
    > The hand-waving was around how to generalize it into the driver-model,
    > or PM QoS. We're already doing this for OMAP, but in an OMAP-specific
    > way, but it's become clear that this is something useful to
    > generalize.

    Do you have a pointer to the source and description? It might be useful
    to look at to do a reality check on what we're talking about.

    James


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