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    SubjectRe: [PATCH 2/2] ACPI / hotplug / driver core: Handle containers in a special way
    On Fri, Dec 27, 2013 at 11:28:34PM +0100, Rafael J. Wysocki wrote:
    > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    >
    > ACPI container devices require special hotplug handling, at least
    > on some systems, since generally user space needs to carry out
    > system-specific cleanup before it makes sense to offline devices in
    > the container. However, the current ACPI hotplug code for containers
    > first attempts to offline devices in the container and only then it
    > notifies user space of the container offline.
    >
    > Moreover, after commit 202317a573b2 (ACPI / scan: Add acpi_device
    > objects for all device nodes in the namespace), ACPI device objects
    > representing containers are present as long as the ACPI namespace
    > nodes corresponding to them are present, which may be forever, even
    > if the container devices are physically detached from the system (the
    > return values of the corresponding _STA methods change in those
    > cases, but generally the namespace nodes themselves are still there).
    > Thus it is useful to introduce entities representing containers that
    > will go away during container hot-unplug.
    >
    > The goal of this change is to address both the above issues.
    >
    > The idea is to create a "companion" container system device for each
    > of the ACPI container device objects during the initial namespace
    > scan or on a hotplug event making the container present. That system
    > device will be unregistered on container removal. A new bus type
    > for container devices is added for this purpose, because device
    > offline and online operations need to be defined for them. The
    > online operation is a trivial function that is always successful
    > and the offline uses a callback pointed to by the container device's
    > offline member.
    >
    > For ACPI containers that callback simply walks the list of ACPI
    > device objects right below the container object (its children) and
    > checks if all of their physical companion devices are offline. If
    > that's not the case, it returns -EBUSY and the container system
    > device cannot be put offline. Consequently, to put the container
    > system device offline, it is necessary to put all of the physical
    > devices depending on its ACPI companion object offline beforehand.
    >
    > Container system devices created for ACPI container objects are
    > initially online. They are created by the container ACPI scan
    > handler whose hotplug.demand_offline flag is set. That causes
    > acpi_scan_hot_remove() to check if the companion container system
    > device is offline before attempting to remove an ACPI container or
    > any devices below it. If the check fails, a KOBJ_CHANGE uevent is
    > emitted for the container system device in question and user space
    > is expected to offline all devices below the container and the
    > container itself in response to it. Then, user space can finalize
    > the removal of the container with the help of its ACPI device
    > object's eject attribute in sysfs.
    >
    > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    > Tested-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>

    Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


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