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

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Message-ID: <20140704145752.GJ6501@e106497-lin.cambridge.arm.com>
Date:	Fri, 4 Jul 2014 15:57:52 +0100
From:	Liviu Dudau <Liviu.Dudau@....com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	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 07, 2014 at 11:44:51PM +0100, Bjorn Helgaas wrote:
> 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,

I'm coming around to your way of thinking and I want to suggest a strategy
for adding the domain number into the PCI framework.

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.

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.

Did I understood correctly this time your intentions? Do you agree with
this plan?

Best regards,
Liviu


> 
> Bjorn
> --
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