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    SubjectPort control interface (was Re: [PATCH] ata: ahci: power off unused ports)
    Mark Lord wrote:
    > And a more general note: I still believe we should have a follow-up
    > feature to this one, to enable polling for newly inserted drives.
    >
    > That would allow powering down idle ports to save money/planet/whatever,
    > but still with hotplug capability. The polling interval should be
    > tunable in /sys, with a default of, say, once every couple of seconds.
    >
    > Thanks for working on this stuff.


    In the grand open source tradition of "pass the buck", I've long been
    hoping that someone would take a few days, sit down, and hammer out the
    policy side of this.

    We -don't- want to do hotplug-active all the time -- the current default
    in all drivers that support device hotplug -- because it needlessly
    keeps parts active that are unused 99.9% of the time [when they are empty].

    Admins need a generic way to control SATA ports and links from
    userspace. Within that, admins need to be able to set a link's hotplug
    state among three choices: hotplug-active [when supported],
    hotplug-poll, and hotplug-never. And of course, hotplug-poll's interval
    should be tunable.

    And of course this control interface needs to be usable on SAS/SATA
    cards that will be common in the future (broadsas, mvsas are early
    examples).


    This is why I look askance at an AHCI BIOS flag. That's merely a hint,
    and potentially unreliable one at that. It could just be describing
    what the BIOS writer felt were the ports that _should_ be hotpluggable
    -- i.e. not even describing what is _possible_ but someone's definition
    of "reasonable." The BIOS flag might even be filled with fuzz (AHCI's
    BIOS-written registers have occasionally been shipped into the field
    uninitialized).

    A better solution involves taking the BIOS flag and using that to set a
    default policy that an admin can easily override using the
    above-mentioned control interface.

    Because in some cases, that BIOS flag might do the wrong thing, and we
    need to give the admin an ability to undo the damage.

    Jeff





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