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    SubjectRe: [PATCH v7 4/6] pci: Introduce a domain number for pci_host_bridge.
    On Fri, Jul 4, 2014 at 8:57 AM, Liviu Dudau <Liviu.Dudau@arm.com> wrote:
    > On Mon, Apr 07, 2014 at 11:44:51PM +0100, Bjorn Helgaas wrote:
    >> On Mon, Apr 7, 2014 at 4:07 AM, Liviu Dudau <Liviu.Dudau@arm.com> wrote:
    >> > On Mon, Apr 07, 2014 at 10:14:18AM +0100, Benjamin Herrenschmidt wrote:
    >> >> On Mon, 2014-04-07 at 09:46 +0100, Liviu Dudau wrote:
    >> >> >
    >> >> > *My* strategy is to get rid of pci_domain_nr(). I don't see why we need
    >> >> > to have arch specific way of providing the number, specially after looking
    >> >> > at the existing implementations that return a value from a variable that
    >> >> > is never touched or incremented. My guess is that pci_domain_nr() was
    >> >> > created to work around the fact that there was no domain_nr maintainance in
    >> >> > the generic code.
    >> >>
    >> >> Well, there was no generic host bridge structure. There is one now, it should
    >> >> go there.
    >> >
    >> > Exactly! Hence my patch. After it gets accepted I will go through architectures
    >> > and remove their version of pci_domain_nr().
    >>
    >> Currently the arch has to supply pci_domain_nr() because that's the
    >> only way for the generic code to learn the domain. After you add
    >> pci_create_root_bus_in_domain(), the arch can supply the domain that
    >> way, and we won't need the arch-specific pci_domain_nr(). Right?
    >> That makes more sense to me; thanks for the explanation.
    >>
    >> Let me try to explain my concern about the
    >> pci_create_root_bus_in_domain() interface. We currently have these
    >> interfaces:
    >>
    >> pci_scan_root_bus()
    >> pci_scan_bus()
    >> pci_scan_bus_parented()
    >> pci_create_root_bus()
    >>
    >> pci_scan_root_bus() is a higher-level interface than
    >> pci_create_root_bus(), so I'm trying to migrate toward it because it
    >> lets us remove a little code from the arch, e.g., pci_scan_child_bus()
    >> and pci_bus_add_devices().
    >>
    >> I think we can only remove the arch-specific pci_domain_nr() if that
    >> arch uses pci_create_root_bus_in_domain(). When we convert an arch
    >> from using scan_bus interfaces to using
    >> pci_create_root_bus_in_domain(), we will have to move the rest of the
    >> scan_bus code (pci_scan_child_bus(), pci_bus_add_devices()) back into
    >> the arch code.
    >>
    >> One alternative is to add an _in_domain() variant of each of these
    >> interfaces, but that doesn't seem very convenient either. My idea of
    >> passing in a structure would also require adding variants, so there's
    >> not really an advantage there, but I am thinking of the next
    >> unification effort, e.g., for NUMA node info. I don't really want to
    >> have to change all the _in_domain() interfaces to also take yet
    >> another parameter for the node number.
    >
    > ...
    > My understanding is that when pci_host_bridge structure was introduced
    > you were trying to keep the APIs unchanged and hence the creation of a
    > bridge was hidden inside the pci_create_root_bus() function.

    You mean pci_alloc_host_bridge()? Right; ideally I would have used
    pci_scan_root_bus() everywhere and gotten rid of pci_create_root_bus().
    The outline of pci_scan_root_bus() is:

    pci_create_root_bus()
    pci_scan_child_bus()
    pci_bus_add_devices()

    The problem was that several arches do interesting things scattered among
    that core. The ACPI host bridge driver used on x86 and ia64 does resource
    allocation before pci_bus_add_devices(), as does parisc. Probably all
    arches should do this, but they don't.

    And powerpc and sparc use of_scan_bus() or something similar instead of
    pci_scan_child_bus(). They probably *could* provide config space accessors
    that talk to OF and would allow pci_scan_child_bus() to work. But that
    seemed like too much work at the time.

    > If we want to store the domain_nr information in the host bridge structure,
    > together with a pointer to sysdata, then we need to break up the creation
    > of the pci_host_bridge from the creation of a root bus. At that moment,
    > pci_scan_root_bus() will need to be changed to accept a pci_host_bridge
    > pointer, while pci_scan_bus() and pci_scan_bus_parented() will create
    > the host bridge in the body of their function.

    It's hard to change an existing interface like pci_scan_root_bus() because
    it's called from so many places and you have to change them all at once.
    Then if something goes wrong, the revert makes a mess for everybody. But
    I think it makes sense to add a new interface that does what you want.

    Bjorn


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