Re: [PATCH v10 07/10] OF: Introduce helper function for getting PCI domain_nr

[Arnd Bergmann]

On Tuesday 09 September 2014 09:46:21 Liviu Dudau wrote:
> On Tue, Sep 09, 2014 at 06:54:21AM +0100, Yijing Wang wrote:
> > >>> on new requests. This function gets called quite a lot and I'm trying not to
> > >>> make it too heavy weight.
> > >>
> > >> Generally, nothing should be accessing the same DT value frequently.
> > >> It should get cached somewhere.
> > >>
> > > 
> > > The problem appears for DTs that don't have the pci-domain info. Then the cached
> > > value is left at the default non-valid value and attempts to rescan the DT will
> > > be made every time the function is called.
> > > 
> > >> I don't really understand how domains are used so it's hard to provide
> > >> a recommendation here. Do domains even belong in the DT?
> > > 
> > > ACPI calls them segments and the way Bjorn explained it to me at some moment was
> > > that it was an attempt to split up a bus in different groups (or alternatively,
> > > merge a few busses together). To be honest I haven't seen systems where the domain
> > > is anything other than zero, but JasonG (or maybe Benjamin) were floating an
> > > idea of using the domain number to identify physical slots.
> > 
> > PCI domain(or named segment) is provided by firmware, in ACPI system, we evaluated it
> > by method "_SEG". in IA64 with ACPI, PCI hostbridge driver retrieves the domain from ACPI,
> > if it's absent, the default domain is zero. So I wonder why in DTS, if it's absent, we get
> > a auto increment domain value.
> 
> Because you can have more than one hostbridge (rare, but not impossible) and unless you
> want to join the two segments, you might want to give it a different domain.

I think you misunderstood the question. The difference is that in ACPI you
are required to specify the domain, while in DT it is optional with your
implementation.

I think in general it would be nice if we could mandate that in DT you also
have to always provide a domain number, however the problem is that we can't
change the existing DTB files that people are using that do not specify a
domain.

We could possibly make this an architecture specific setting though and
mandate that all ARM64 platforms have to set it, while ARM32 does not need
it.

	Arnd

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