Re: [PATCH 2/3] ACPI / hotplug: Fix PCI host bridge hot removal

[Toshi Kani]

On Wed, 2013-11-20 at 01:08 +0100, Rafael J. Wysocki wrote:
> On Wednesday, November 20, 2013 12:42:28 AM Rafael J. Wysocki wrote:
> > On Tuesday, November 19, 2013 02:58:40 PM Toshi Kani wrote:
> > > On Tue, 2013-11-19 at 22:10 +0100, Rafael J. Wysocki wrote:
> > > > > On Tuesday, November 19, 2013 10:48:51 AM Toshi Kani wrote:
> > > > > > On Mon, 2013-11-18 at 22:39 +0100, Rafael J. Wysocki wrote:
> > > > > > > On Monday, November 18, 2013 11:10:05 AM Toshi Kani wrote:
> > > > > > > > On Thu, 2013-11-14 at 00:16 +0100, Rafael J. Wysocki wrote:
> > > > > > > > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > > > > > > > > 
> > > > > > > > > Since the PCI host bridge scan handler does not set hotplug.enabled,
> > > > > > > > > the check of it in acpi_bus_device_eject() effectively prevents the
> > > > > > > > > root bridge hot removal from working after commit a3b1b1ef78cd
> > > > > > > > > (ACPI / hotplug: Merge device hot-removal routines).  However, that
> > > > > > > > > check is not necessary, because the other acpi_bus_device_eject()
> > > > > > > > > users, acpi_hotplug_notify_cb and acpi_eject_store(), do the same
> > > > > > > > > check by themselves before executing that function.
> > > > > > > > > 
> > > > > > > > > For this reason, remove the scan handler check from
> > > > > > > > > acpi_bus_device_eject() to make PCI hot bridge hot removal work
> > > > > > > > > again.
> > > > > > > > 
> > > > > > > > I am curious why the PCI host bridge scan handler does not set
> > > > > > > > hotplug.enabled.  Is this how it disables hotplug via sysfs eject but
> > > > > > > > enables via ACPI notification?
> > > > > > > 
> > > > > > > It just doesn't register for hotplug at all.  I guess it could set that
> > > > > > > bit alone, but then it would be quite confusing and the check is not
> > > > > > > necessary anyway.
> > > > > > 
> > > > > > I see.  Given how the PCI host bridge scan handler is integrated today,
> > > > > > the change looks reasonable to me.
> > > > > 
> > > > > Looking further, I noticed that there is one more issue to address.  The
> > > > > patch below applies on top of your patchset.
> > > > >
> > > > > From: Toshi Kani <toshi.kani@xxxxxx>
> > > > > Subject: [PATCH] ACPI / hotplug: Fix conflicted PCI bridge notify
> > > > > handlers
> > > > > 
> > > > > The PCI host bridge scan handler installs its own notify handler,
> > > > > handle_hotplug_event_root(), by itself.  Nevertheless, the ACPI
> > > > > hotplug framework also installs the common notify handler,
> > > > > acpi_hotplug_notify_cb(), for PCI root bridges.  This causes
> > > > > acpi_hotplug_notify_cb() to call _OST method with unsupported
> > > > > error as hotplug.enabled is not set.
> > > > > 
> > > > > To address this issue, introduce hotplug.self_install flag, which
> > > > > indicates that the scan handler installs its own notify handler by
> > > > > itself.  The ACPI hotplug framework does not install the common
> > > > > notify handler when this flag is set.
> > > > 
> > > > Good catch!
> > > > 
> > > > Still, I don't think we need a new flag, because we know that that
> > > > scan handler doesn't support hotplug, because its hotplug profile
> > > > hasn't been registered (that actually applies to all scan handlers
> > > > without hotplug support, not only the root host bridge).  
> > > 
> > > When a scan handler does not support hotplug at all, the common notify
> > > handler should be installed so that it can call _OST with an appropriate
> > > response.
> > 
> > That creates an arbitrary difference between devices that have scan handlers
> > and devices that don't have them (PCI, USB, SATA etc).  So if we want _OST
> > to be called for all devices for which hotplug is not supported, that
> > should be implemented in a different way and not necessarily in 3.13.

I do not think we have an immediate issue since it only matters when the
firmware supports ACPI hotplug with _OST.  Such device types are CPU,
memory, container, and PCI bridge, which uses scan handlers.  USB, SATA,
etc. do not use ACPI hotplug.  That said, ideally, we should be able to
call _OST for any device types.
 
> > > > Moreover,
> > > > if it does support hotplug, but the hotplug profile hasn't been
> > > > registered due to an error, we still should not install the notify
> > > > handler I think.  
> > > 
> > > This case, I think the common notify handler should be installed so that
> > > it can call _OST for error response as well.  The question is what to do
> > > when a scan handler has its own notify handler.
> 
> Actually, having considered this particular case a bit more I think that it
> is useful to install acpi_hotplug_notify_cb() for things whose scan handlers
> register hotplug support, but the registration fails (which should be treated
> as "permanently disabled").
>
> However, I still think that devices whose scan handlers don't support hotplug
> at all should be treated consistently with devices without scan handlers.

Basically, the kernel needs to be compliant with ACPI spec.  Once the
kernel tells firmware that it supports _OST, it needs to call _OST for
an ACPI hotplug event when _OST is implemented on the object.  Although
we cannot assure this behavior for the devices without scan handler, we
should do so for the devices with scan handlers.

> So, what about the slightly modified patch below?

Well, I do not think it is right.  The kernel is supposed to tell
firmware that it does not support hotplug when it doesn't... 

Thanks,
-Toshi

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