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

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Message-ID: <13221031.pZI6l8TTGT@vostro.rjw.lan>
Date:	Wed, 20 Nov 2013 12:56:45 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Toshi Kani <toshi.kani@...com>
Cc:	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PCI <linux-pci@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: [PATCH 2/3] ACPI / hotplug: Fix PCI host bridge hot removal

On Tuesday, November 19, 2013 06:22:07 PM Toshi Kani wrote:
> 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@...el.com>
> > > > > > > > > > 
> > > > > > > > > > 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@...com>
> > > > > > 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... 

OK, I see your point.

I'll apply your patch, then, but I'm going to rename the new flag to
"ignore".

Thanks!

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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