Re: [RFC PATCH 00/16] Refine PCI host bridge scan interfaces

[Tomasz Nowicki]

On 20.11.2014 13:08, Liviu Dudau wrote:
> On Thu, Nov 20, 2014 at 12:54:48PM +0100, Tomasz Nowicki wrote:
> > On 17.11.2014 15:13, Arnd Bergmann wrote:
> > > On Monday 17 November 2014 18:21:34 Yijing Wang wrote:
> > > > This series is based Linux 3.18-rc1 and Lorenzo Pieralisi's
> > > > arm PCI domain cleanup patches, link:
> > > > https://patchwork.ozlabs.org/patch/407585/
> > > >
> > > > Current pci scan interfaces like pci_scan_root_bus() and directly
> > > > call pci_create_root_bus()/pci_scan_child_bus() lack flexiblity.
> > > > Some platform infos like PCI domain and msi_chip have to be
> > > > associated to PCI bus by some arch specific function.
> > > > We want to make a generic pci_host_bridge, and make it hold
> > > > the platform infos or hook. Then we could eliminate the lots
> > > > of arch pci_domain_nr, also we could associate some platform
> > > > ops something like pci_get_msi_chip(struct pci_dev *dev)
> > > > with pci_host_bridge to avoid introduce arch weak functions.
> > > >
> > > > This RFC version not for all platforms, just applied the new
> > > > scan interface in x86/arm/powerpc/ia64, I will refresh other
> > > > platforms after the core pci scan interfaces are ok.
> > >
> > > I think overall this is a good direction to take, in particular
> > > moving more things into struct pci_host_bridge so we can
> > > slim down the architecture specific code.
> > >
> > > I don't particularly like the way you use the 'pci_host_info'
> > > to pass callback pointers and some of the generic information.
> > > This duplicates some of the issues we are currently trying
> > > to untangle in the arm32 code to make drivers easier to share
> > > between architectures.
> > >
> > > As a general approach, I'd rather see generic helper functions
> > > being exported by the PCI core that a driver may or may not
> > > call.
> > > The way you split the interface between things that happen
> > > before scanning the buses (pci_create_host_bridge) and
> > > the actual scanning (__pci_create_root_bus, pci_scan_child_bus)
> > > seems very helpful and I think we can expand that concept further:
> > >
> > > - The normal pci_create_host_bridge() function can contain
> > >    all of the DT scanning functions (finding bus/mem/io resources,
> > >    finding the msi-parent), while drivers that don't depend on DT
> > >    for this information can call the same function and fill the
> > >    same things after they have the pci_host_bridge pointer.
> >
> > How about finding PCI domain number (in the DT way) within
> > pci_create_host_bridge() too ?
>
> It is an idea worth pursuing for the 99% of the cases. I would like
> to understand the 1% of the time when we want a domain number to be
> shared between two host bridges or the time when we want more than
> one domain per bridge.
Even though we have shared domain, this should be resolved via DT calls, 
do I miss something ?

> Is that possible? Is it useful? Is it already in practice?
This is good question... IMO:
1. Two host bridges can shared domain number if they are children of the 
same parent host bridge.
2. But I can not find good explanation for more than one domain per bridge.

Tomasz

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