On Mon, Sep 17, 2012 at 10:24 AM, Bjorn Helgaas <bhelgaas@xxxxxxxxxx> wrote:
> On Mon, Sep 17, 2012 at 9:55 AM, Jiang Liu <liuj97@xxxxxxxxx> wrote:
>> On 09/14/2012 01:40 AM, Bjorn Helgaas wrote:
>>> On Thu, Sep 13, 2012 at 10:00 AM, Jiang Liu <liuj97@xxxxxxxxx> wrote:
>>>> This patch introduces two root bridge hotplug safe interfaces to walk
>>>> all root buses. Function pci_get_root_buses() takes a snopshot of the
>>>> pci_root_buses list and holds a reference count to each root buses.
>>>> pci_{get|put}_root_buses are used to replace hotplug unsafe interface
>>>> pci_find_next_bus().
>>>
>>> Honestly, I think the whole idea of walking these lists is wrong, and
>>> adding safer interfaces just perpetuates the idea that it's OK to walk
>>> them.
>>>
>>> We should be doing the setup in the device add path instead. I know
>>> we have other issues with that in some cases, but I'd like to at least
>>> move in that direction.
>>>
>>> For example, sba_init() is a problem because it's an ACPI driver, and
>>> we currently enumerate PCI devices before binding most ACPI drivers.
>>> That's broken -- in that particular case, there's an HWP0001 IOMMU
>>> device that encloses the PNP0A03 PCI host bridge. Currently we bind
>>> the PNP0A03 driver first, enumerate the PCI devices below it, then
>>> bind the HWP0001 driver (sba_init). Obviously that's backwards and
>>> the HWP0001 driver should have been bound first, then the PNP0A03
>>> driver. But I don't think we're ready to make that shift yet (though
>>> it'd be nice if somebody were working on it).
>> I remember there were some discussions on the mail list above the divergence
>> between boot and hotplug paths. But it's a little hard for me to work on
>> this, I only have experience with PCI on IA64 and x86:(
>>
>>>
>>> I wonder if we could add some kind of iterator that does the list
>>> traversals in the PCI core and calls a callback for every device? I
>>> think that would work for sba_init(), but I don't know about the
>>> others. This would still be ugly in that the iterator would have to
>>> hold some sort of hotplug lock while doing for_each_pci_dev() and the
>>> callers, e.g., sba_init(), are not solving the problem for hot-added
>>> devices, but at least the locking would be in the core and the drivers
>>> would stop depending on the lists themselves.
>
>> I will try the iterator first, hope we could find a solution here.
>
> A plain iterator only handles devices that already exist. But I
> wonder if it would work to have an interface like "call this callback
> for every device that exists already *and* for every device that's
> hot-added in the future." The bus notifiers are close to this, e.g.,
> "bus_register_notifier(&pci_bus_type, ...)" handles this for hot-added
> devices. A little glue around it could take care of doing it for
> already-existing devices as well.
BTW, while reviewing Yinghai's vga patch, I found a case in
vga_arb_device_init() that does exactly this: registers a notifier to
catch future hot-added devices, then calls the notifier "add" function
for every existing device. So that's another place that could use
something like this.
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