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

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Date:	Tue, 19 Nov 2013 14:58:40 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
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 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.

> 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.

> So, I prefer the patch below.
>
> And this fix will remain useful after my recent series at
> http://marc.info/?l=linux-pci&m=138470560909690&w=4

I agree that the PCI host bridge scan handler should use the common
notify handler, and your change is on the right direction.  But as I
understand, this patchset is for 3.14.  For 3.13, I thought we have to
live with the separate PCI root notify handler, and this patch avoids
the conflict for the time being.  If necessary, the new flag can be
removed in 3.14.

Thanks,
-Toshi

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