Re: [RFC PATCH v3 0/3] acpi: Introduce prepare_remove deviceoperation

[Toshi Kani]

On Tue, 2012-12-04 at 17:16 +0800, Hanjun Guo wrote:
> On 2012/12/4 8:10, Toshi Kani wrote:
> > On Mon, 2012-12-03 at 12:25 +0800, Hanjun Guo wrote:
> >> On 2012/11/30 6:27, Toshi Kani wrote:
> >>> On Thu, 2012-11-29 at 12:48 +0800, Hanjun Guo wrote:
> >>>> On 2012/11/29 2:41, Toshi Kani wrote:
> >>>>> On Wed, 2012-11-28 at 19:05 +0800, Hanjun Guo wrote:
> >>>>>> On 2012/11/24 1:50, Vasilis Liaskovitis wrote:
> >>>>>> As you may know, the ACPI based hotplug framework we are working on already addressed
> >>>>>> this problem, and the way we slove this problem is a bit like yours.
> >>>>>>
> >>>>>> We introduce hp_ops in struct acpi_device_ops:
> >>>>>> struct acpi_device_ops {
> >>>>>> 	acpi_op_add add;
> >>>>>> 	acpi_op_remove remove;
> >>>>>> 	acpi_op_start start;
> >>>>>> 	acpi_op_bind bind;
> >>>>>> 	acpi_op_unbind unbind;
> >>>>>> 	acpi_op_notify notify;
> >>>>>> #ifdef	CONFIG_ACPI_HOTPLUG
> >>>>>> 	struct acpihp_dev_ops *hp_ops;
> >>>>>> #endif	/* CONFIG_ACPI_HOTPLUG */
> >>>>>> };
> >>>>>>
> >>>>>> in hp_ops, we divide the prepare_remove into six small steps, that is:
> >>>>>> 1) pre_release(): optional step to mark device going to be removed/busy
> >>>>>> 2) release(): reclaim device from running system
> >>>>>> 3) post_release(): rollback if cancelled by user or error happened
> >>>>>> 4) pre_unconfigure(): optional step to solve possible dependency issue
> >>>>>> 5) unconfigure(): remove devices from running system
> >>>>>> 6) post_unconfigure(): free resources used by devices
> >>>>>>
> >>>>>> In this way, we can easily rollback if error happens.
> >>>>>> How do you think of this solution, any suggestion ? I think we can achieve
> >>>>>> a better way for sharing ideas. :)
> >>>>>
> >>>>> Yes, sharing idea is good. :)  I do not know if we need all 6 steps (I
> >>>>> have not looked at all your changes yet..), but in my mind, a hot-plug
> >>>>> operation should be composed with the following 3 phases.
> >>>>
> >>>> Good idea ! we also implement a hot-plug operation in 3 phases:
> >>>> 1) acpihp_drv_pre_execute
> >>>> 2) acpihp_drv_execute
> >>>> 3) acpihp_drv_post_execute
> >>>> you may refer to :
> >>>> https://lkml.org/lkml/2012/11/4/79
> >>>
> >>> Great.  Yes, I will take a look.
> >>
> >> Thanks, any comments are welcomed :)
> > 
> > If I read the code right, the framework calls ACPI drivers differently
> > at boot-time and hot-add as follows.  That is, the new entry points are
> > called at hot-add only, but .add() is called at both cases.  This
> > requires .add() to work differently.
> 
> Hi Toshi,
> Thanks for your comments!
> 
> > 
> > Boot    : .add()
> 
> Actually, at boot time: .add(), .start()

Right.

> > Hot-Add : .add(), .pre_configure(), configure(), etc.
> 
> Yes, we did it as you said in the framework. We use .pre_configure(), configure(),
> and post_configure() to instead of .start() for better error handling and recovery.

I think we should have hot-plug interfaces at the module level, not at
the ACPI-internal level.  In this way, the interfaces can be
platform-neutral and allow any modules to register, which makes it more
consistent with the boot-up sequence.  It can also allow ordering of the
sequence among the registered modules.  Right now, we initiate all
procedures from ACPI during hot-plug, which I think is inflexible and
steps into other module's role.

I am also concerned about the slot handling, which is the core piece of
the infrastructure and only allows hot-plug operations on ACPI objects
where slot objects are previously created by checking _EJ0.  The
infrastructure should allow hot-plug operations on any objects, and it
should not be dependent on the slot design.

I have some rough idea, and it may be easier to review / explain if I
make some code changes.  So, let me prototype it, and send it you all if
that works out.  Hopefully, it won't take too long.

> > I think the boot-time and hot-add initialization should be done
> > consistently.  While there is difficulty with the current boot sequence,
> > the framework should be designed to allow them consistent, not make them
> > diverged.
> > 
> >>>>> 1. Validate phase - Verify if the request is a supported operation.  All
> >>>>> known restrictions are verified at this phase.  For instance, if a
> >>>>> hot-remove request involves kernel memory, it is failed in this phase.
> >>>>> Since this phase makes no change, no rollback is necessary to fail. 
> >>>>
> >>>> Yes, we have done this in acpihp_drv_pre_execute, and check following things:
> >>>>
> >>>> 1) Hot-plugble or not. the instance kernel memory you mentioned is also checked
> >>>>    when memory device remove;
> >>>
> >>> Agreed.
> >>>
> >>>> 2) Dependency check involved. For instance, if hot-add a memory device,
> >>>>    processor should be added first, otherwise it's not valid to this operation.
> >>>
> >>> I think FW should be the one that assures such dependency.  That is,
> >>> when a memory device object is marked as present/enabled/functioning, it
> >>> should be ready for the OS to use.
> >>
> >> Yes, BIOS should do something for the dependency, because BIOS knows the
> >> actual hardware topology. 
> > 
> > Right.
> > 
> >> The ACPI specification provides _EDL method to
> >> tell OS the eject device list, but still has no method to tell OS the add device
> >> list now.
> > 
> > Yes, but I do not think the OS needs special handling for add...
> 
> Hmm, how about trigger a hot add operation by OS ? we have eject interface for OS, but
> have no add interface now, do you think this feature is useful? If it is, I think OS
> should analyze the dependency first and tell the user.

The OS can eject an ACPI device because a target device is owned by the
OS (i.e. enabled).  For hot-add, a target ACPI device is not owned by
the OS (i.e. disabled).  Therefore, the OS is not supposed to change its
state.  So, I do not think we should support a hot-add operation by the
OS.
 
> >> For some cases, OS should analyze the dependency in the validate phase. For example,
> >> when hot remove a node (container device), OS should analyze the dependency to get
> >> the remove order as following:
> >> 1) Host bridge;
> >> 2) Memory devices;
> >> 3) Processor devices;
> >> 4) Container device itself;
> > 
> > This may be off-topic, but how do you plan to delete I/O devices under a
> > node?  Are you planning to delete all I/O devices along with the node?
> 
> Yes, we delete all I/O devices under the node. we delete I/O devices as
> following steps:
> 1) Offline PCI devices;
> 2) Offline IOAPIC and IOMMU;
> and offline I/O devices no matter in use or not.

Oh, off-lining no matter what would be problematic for enterprise
customers... 
 
> > On other OS, we made a separate step called I/O chassis delete, which
> > off-lines all I/O devices under the node, and is required before a node
> > hot-remove.  It basically triggers PCIe hot-remove to detach drivers
> > from all devices.  It does not eject the devices so that they do not
> > have to be on hot-plug slots.  This step runs user-space scripts to
> > verify if the devices can be off-lined without disrupting user's
> > applications, and provides comprehensive reports if any of them are in
> 
> Great! we also have a plan to implement this feature.

That's great!

> > use.  Not sure if Linux's PCI hot-remove has such check, but I thought
> > I'd mention it. :)
> 
> Have no such check, I'm sure :)
> 
> > 
> >> In this way, we can check that all the devices are hot-plugble or not under the
> >> container device before execute phase, and further more, we can remove devices
> >> in order to avoid some crash problems.
> > 
> > Yes, we should check if all the resources under the node can be
> > off-lined at validate phase.  (note, all the devices do not have to have
> > _EJ0 if that's what you meant by hot-pluggable.)
> 
> Yes, agreed. For node hotplug, no need for all the devices have _EJ0 method.

Right.

Thanks,
-Toshi


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