linux-kernel - Re: [PATCH v7 4/6] pci: Introduce a domain number for pci_host

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Date:	Mon, 7 Apr 2014 16:44:51 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	linux-pci <linux-pci@...r.kernel.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Arnd Bergmann <arnd@...db.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	LAKML <linux-arm-kernel@...ts.infradead.org>,
	Tanmay Inamdar <tinamdar@....com>,
	Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [PATCH v7 4/6] pci: Introduce a domain number for pci_host_bridge.

On Mon, Apr 7, 2014 at 4:07 AM, Liviu Dudau <Liviu.Dudau@....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.

Bjorn
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