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SubjectRe: [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 <> wrote:
> On Wed, Feb 6, 2013 at 11:59 AM, Yinghai Lu <> wrote:
>> On Wed, Feb 6, 2013 at 9:54 AM, Bjorn Helgaas <> 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.


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.



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