linux-kernel - Re: [PATCH v3 00/22] PCI: Iterate pci host bridge instead of pci root bus

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Message-ID: <CAE9FiQX=uEPjHDi-_VaW88ueH4u9C22gmDiGQewsQ=VN5T4=Gg@mail.gmail.com>
Date:	Wed, 6 Feb 2013 13:28:27 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Jiang Liu <jiang.liu@...wei.com>,
	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Tony Luck <tony.luck@...el.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Taku Izumi <izumi.taku@...fujitsu.com>,
	Toshi Kani <toshi.kani@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-pci@...r.kernel.org, Russell King <linux@....linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 00/22] PCI: Iterate pci host bridge instead of pci root bus

On Wed, Feb 6, 2013 at 12:50 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Wed, Feb 6, 2013 at 11:59 AM, Yinghai Lu <yinghai@...nel.org> wrote:
>> On Wed, Feb 6, 2013 at 9:54 AM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
>>> I think you're missing the point.
>>>
>>> Search the tree for uses of "for_each_pci_dev()."  Almost every
>>> occurrence is a bug because that code doesn't work correctly for
>>> hot-added devices.  That code gets run for devices that are present at
>>> boot, but not for devices hot-added later.
>>>
>>> You're proposing to add "for_each_pci_host_bridge()."  That will have
>>> the exact same problem as for_each_pci_dev() already does.  Code that
>>> uses it will work for host bridges present at boot, but not for
>>> bridges hot-added later.
>>>
>>> Why would we want to add an interface when every use of that interface
>>> will be a design bug?  I think we should fix the existing users of
>>> pci_root_buses by changing their designs so they will work correctly
>>> with host bridge hotplug.
>>
>> I'm a little confused about what you want.
>>
>> In boot stage using for_each_pci_host_bridge or pci_root_buses is fine.
>
> No, it's not.  Boot-time should work exactly the same way as hot-add
> time, unless there's a reason it can't.  There might be corner cases
> where we can't do that yet, e.g., the pcibios_resource_survey stuff,
> but in general I don't think there's anything that *forces* us to
> iterate through host bridges at boot-time.
>
>> For those cases that it should support host-bridge by nature.
>> there are two solutions:
>> 1. use for_each_pci_host_bridge, and it is hotplug-safe.
>
> I'm trying to tell you that for_each_pci_host_bridge() is NOT
> hotplug-safe.  Your series makes it safer than it was, in the sense
> that it probably fixes stray pointer references when a host bridge
> hotplug happens while somebody's traversing pci_root_buses.  But the
> whole point of for_each_pci_host_bridge() is to run some code for
> every bridge we know about *right now*.  If a host bridge is added
> right after the for_each_pci_host_bridge() loop exits, that code
> doesn't get run.  In most cases, that's a bug.  The only exception I
> know about is the /sys/.../rescan interface.
>
>> and make sgi_hotplug to use acpi_pci_driver interface.
>> and acpi_pci_root_add() will call .add in the acpi_pci_driver.
>>
>> 2. make them all to be built-in, and  those acpi_pci_driver should be registered
>> much early before acpi_pci_root_add is called.
>> then we don't need to call for_each_host_bridge for it.
>>
>> So difference between them:
>> 1. still keep the module support, and register acpi_pci_driver later.
>> 2. built-in support only, and need to register acpi_pci_driver early.
>
> acpi_pci_driver is going away, so I don't want to add any more uses of
> it.  Obviously it's only relevant to x86 and ia64 anyway.

when?

I have ioapic and iommu hotplug patch set that need to use them.

>
> What I'd like is a change where for_each_pci_host_bridge() is used
> only inside the PCI core and defined somewhere like drivers/pci/pci.h
> so it's not even available outside.
>
> The other uses should be changed so they use
> pcibios_root_bridge_prepare() or something similar.  That way, it will
> be obvious that the code supports hot-added bridges, and when it gets
> copied to other places, we'll be copying a working design instead of a
> broken one.
>
> I don't want to have to audit these places and figure out "this is
> safe because this arch doesn't support host bridge hotplug" or "this
> is safe because this CPU doesn't support XYZ."  That's not a
> maintainable solution.  The safety should be apparent from the code
> itself, without requiring extra platform-specific knowledge.

so you still did not answer you want 1 or 2 yet:

for sgi_hotplug,

1. still keep the module support, and register acpi_pci_driver later.
2. built-in support only, and need to register acpi_pci_driver early.

Thanks

Yinghai
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