Re: [PATCH v7 4/6] pci: Introduce a domain number for pci_host_bridge.

On Tue, Jul 08, 2014 at 02:11:36AM +0100, Bjorn Helgaas wrote:

On Fri, Jul 4, 2014 at 8:57 AM, Liviu Dudau [off-list ref] wrote:

quoted

On Mon, Apr 07, 2014 at 11:44:51PM +0100, Bjorn Helgaas wrote:

quoted

On Mon, Apr 7, 2014 at 4:07 AM, Liviu Dudau [off-list ref] wrote:

quoted

On Mon, Apr 07, 2014 at 10:14:18AM +0100, Benjamin Herrenschmidt wrote:

quoted

On Mon, 2014-04-07 at 09:46 +0100, Liviu Dudau wrote:

quoted

*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.

quoted

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.

OK, I understand your concern. It does sort of return us back to the initial
discussion, where you were arguing against adding a new set of functions
for every existing function, but it makes sense from transition point of view.

Best regards,
Liviu

Bjorn

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