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    SubjectRe: [RFC] ACPI scan handlers
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    On Fri, 2013-01-25 at 23:11 +0100, Rafael J. Wysocki wrote:
    > On Friday, January 25, 2013 09:52:21 AM Toshi Kani wrote:
    > > On Thu, 2013-01-24 at 01:26 +0100, Rafael J. Wysocki wrote:
    :
    > > >
    > > > I wonder if anyone is seeing any major problems with this at the high level.
    >
    > First of all, thanks for the response. :-)
    >
    > > I agree that the current model is mess. As shown below, it requires
    > > that .add() at boot-time only performs acpi dev init, and .add() at
    > > hot-add needs both acpi dev init and device on-lining.
    >
    > I'm not sure what you're talking about, though.
    >
    > You seem to be confusing ACPI device nodes (i.e. things represented by struct
    > acpi_device objects) with devices, but they are different things. They are
    > just used to store static information extracted from device objects in the
    > ACPI namespace and to expose those objects (and possibly some of their
    > properties) via sysfs. Device objects in the ACPI namespace are not devices,
    > however, and they don't even need to represent devices (for example, the
    > _SB thing, which is represented by struct acpi_device, is hardly a device).
    >
    > So the role of struct acpi_device things is analogous to the role of
    > struct device_node things in the Device Trees world. In fact, no drivers
    > should ever bind to them and in my opinion it was a grievous mistake to
    > let them do that. But I'm digressing.
    >
    > So, when you're saying "acpi dev", I'm not sure if you think about a device node
    > or a device (possibly) represented by that node. If you mean device node, then
    > I'm not sure what "acpi dev init" means, because device nodes by definition
    > don't require any initialization beyond what acpi_add_single_object() does
    > (and they don't require any off-lining beyod what acpi_device_unregister()
    > does, for that matter). In turn, if you mean "device represented by the given
    > device node", then you can't even say "ACPI device" about it, because it very
    > well may be a PCI device, or a USB device, or a SATA device etc.

    Let me clarify my point with the ACPI memory driver as an example since
    it is the one that has caused a problem in .remove().

    acpi_memory_device_add() implements .add() and does two things below.

    1. Call _CRS and initialize a list of struct acpi_memory_info that is
    attached to acpi_device->driver_data. This step is what I described as
    "acpi dev init". ACPI drivers perform driver-specific initialization to
    ACPI device objects.

    2. Call add_memory() to add a target memory range to the mm module.
    This step is what I described as "on-lining". This step is not
    necessary at boot-time since the mm module has already on-lined the
    memory ranges at early boot-time. At hot-add, however, it needs to call
    add_memory() with the current framework.

    Similarly, acpi_memory_device_remove() implements .remove() and does two
    things below.

    1. Call remove_memory() to offline a target memory range. This step,
    "off-lining", can fail since the mm module may or may not be able to
    delete non-movable ranges. This failure cannot be handled properly and
    causes the system to crash at this point.

    2. Free up the list of struct acpi_memory_info. This step deletes
    driver-specific data from an ACPI device object.


    > That's part of the whole confusion, by the way.
    >
    > If the device represented by an ACPI device node is on a natively enumerated
    > bus, like PCI, then its native bus' init code initializes the device and
    > creates a "physical" device object for it, like struct pci_dev, which is then
    > "glued" to the corresponding struct acpi_device by acpi_bind_one(). Then, it
    > is clear which is which and there's no confusion. The confusion starts when
    > there's no native enumeration and we only have the struct acpi_device thing,
    > because then everybody seems to think "oh, there's no physical device object
    > now, so this must be something different", but the *only* difference is that
    > there is no native bus' init code now and we should still be creating a
    > "physical device" object for the device and we should "glue" it to the
    > existing struct acpi_device like in the natively enumerated case.
    >
    > > It then requires .remove() to perform both off-lining and acpi dev
    > > delete. .remove() must succeed, but off-lining can fail.
    > >
    > > acpi dev online
    > > |========|=========|
    > >
    > > add @ boot
    > > -------->
    > > add @ hot-add
    > > ------------------>
    > > <------------------
    > > remove
    >
    > That assumes that the "driver" is present during boot (i.e. when acpi_bus_scan()
    > is run for the first time), but what if it is not?

    With memory's example, the mm module must be present at boot. The
    system does not boot without it.

    > > Your proposal seems to introduce the following new model. If so, I do
    > > not think it addresses all the issues.
    >
    > It is not supposed to address *all* issues (whatever "all" means). It is meant
    > to address precisely *one* problem, which is the abuse of the driver core by
    > the ACPI subsystem (please see below).
    >
    > > .attach() still needs to behave differently between boot and hot-add.
    >
    > Why does it? I don't see any reason for that.

    With memory's example, calling add_memory() at boot is not necessary
    (which just fails and this failure cannot cause an error), but is
    necessary at hot-add (which should succeed in this case).

    > > The model is also asymmetric since the destructor of .attach() at hot-add
    > > is the combination of .detach() and .untie().
    > > .
    > > attach @ boot
    > > -------->
    > > attach @ hot-add
    > > ----------------->
    > > detach untie
    > > <-------<---------
    > > --------->
    > > reclaim
    > >
    > > I believe device on-lining and off-lining steps should not be performed
    > > in .add() and .remove(). With this clarification, the current .add()
    > > & .remove() model works fine as follows. That is, .add() only performs
    > > acpi dev init, and .remove() only perform acpi dev delete (which is same
    > > as your .detach()). My system device hot-plug framework is designed to
    > > work with this model.
    >
    > Well, if I understand the above correctly, you're basically saying that if we
    > add a layer on top of the ACPI subsystem, we can separate "online" from "add"
    > and "offline" from "remove" in such a way that the "add" and "remove" will be
    > handled by the ACPI subsystem and "online" and "offline" will be done by the
    > extra layer.

    Right. In memory's example, the "online" part should be done by the mm
    module itself.

    > That quite precisely is what we should be doing, but the "add" operation should
    > include the creation of a "physical device" object, like for example struct
    > platform_device, and the additional layer should be a proper driver (a platform
    > driver for example) that will bind to that "physical device" object and
    > initialize the device (i.e. hardware).
    >
    > Analogously, the "remove" operation should include the removal of the "physical
    > device" object from which the driver will have to be unbound first.

    Agreed. With memory's example, the "remove" is also required to do
    "off-lining" (i.e. call remove_memory), which should not be the role of
    ACPI driver.

    > That I believe is what Greg meant when he was discussing your earlier proposal
    > with you.
    >
    > Now, however, the problem is what kind of a device object we should create
    > during the "add" phase (struct platform_device may not be suitable in some
    > cases) and whether that needs to be a single object or a whole bunch of them
    > (e.g. when the given struct acpi_device represents a bus or bridge, like in the
    > PCI host bridge case). That's what the ACPI scan handlers I'm proposing are
    > for.

    OK, so, we are thinking of different issues... :-)

    > So, an ACPI scan handler's .attach() is supposed to recognize what kind of
    > hardware is there to handle and to create whatever device objects (based on
    > struct device) are there to create etc. Then, there should be drivers that
    > will bind to those objects and so on. .detach(), in turn, is supposed to
    > reverse whatever .attach() has done. There is an additional complication,
    > though, that there may be an eject request between .attach() and .detach()
    > and it needs to be responded to.
    >
    > This really is about responding to three types of events related to the ACPI
    > namespace. Those events are, essentially:
    >
    > (1) Device node (i.e. struct acpi_device) has been registered.
    > (2) Eject has been requested for a device node.
    > (3) Device node goes away (i.e. it is going to be unregistered).
    >
    > Whatever the "model", we have to respond to the above events, this way or
    > another.
    >
    > Of course, (2) need not be the same as (3) in general, because one may envision
    > a refusal to carry out the eject. Currently, though, there is no distinction
    > between (2) and (3).
    >
    > The purpose of ACPI scan handlers I'm proposing is precisely to handle these
    > three types of events without abusing the driver core. How exactly they are
    > going to be handled will depend on the implementation of those handlers.
    >
    > The idea is that .attach(), .untie(), and .detach() will be called to handle
    > (1), (2), and (3), respectively, with the additional twist that after an eject
    > refusal .reclaim() needs to be called to do the cleanup.
    >
    > Well, perhaps the names .untie() and .reclaim() are not the best ones and it's
    > better to use names like .eject_requested() and .eject_refused() explicitly
    > for those callbacks? And analogously for the flag indicating that
    > .eject_requested() has succeeded for the given device?
    >
    > So, this is not about creating any new "model", it's just about doing what
    > needs to be done in a possibly straightforward way.
    >
    > Now, perhaps I should just post some code so that it's more clear what I mean. :-)

    Sounds like I did confuse completely!

    Anyway, even we have .untie() or .eject_requested(), I think all the
    hot-delete procedure may not be done within this function since an ACPI
    driver is not responsible for managing/controlling actual device.


    Thanks,
    -Toshi




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