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

[Liviu Dudau]

On Tue, Sep 09, 2014 at 10:30:51AM +0100, Yijing Wang wrote:
> On 2014/9/9 16:46, 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.
> 
> OK. Sorry, I have one last question, because domain will be used to calculate the address used to
> access PCI hardware config registers. So if DTS file doesn't report the domain, how do we know
> we would access the right registers when we use the auto increment domain vaule ?

That's a good question and sides with Arnd's suggestion to try to mandate the presence of the PCI
domain in the DTS. However, by grepping through the source code, it looks like the architectures
that use the domain number for reading config registers (x86-based) are non-DT architectures,
while DT-aware arches seem to ignore the domain number except when printing out messages. Is that
another confirmation that most DT-aware architectures have only run with domain_nr = 0?


> Has there a mechanism to make sure system can access the correct registers by the domain ?

Not as such if you look with x86 glasses. With the exception of powerpc all other architecures
seem to happily assume domain_nr = 0 and ignore it in the computation of configuration registers
offsets.

Best regards,
Liviu

> 
> Thanks!
> Yijing.
> 
> > 
> > Best regards,
> > Liviu
> > 
> >>
> >> PCI get domain by ACPI "_SEG" in IA64(drivers/acpi/pci_root.c)
> >> ......
> >> 	status = acpi_evaluate_integer(handle, METHOD_NAME__SEG, NULL,
> >> 				       &segment);
> >> 	if (ACPI_FAILURE(status) && status != AE_NOT_FOUND) {
> >> 		dev_err(&device->dev,  "can't evaluate _SEG\n");
> >> 		result = -ENODEV;
> >> 		goto end;
> >> 	}
> >> .......
> >>
> >> Thanks!
> >> Yijing.
> >>
> >>>
> >>>> This function
> >>>> is just a weird mixture of data retrieval and allocation. I think you
> >>>> need to separate it into 2 functions.
> >>>
> >>> It is meant to do allocation with the retrieval being a short-cut (or fine
> >>> control if you want).
> >>>
> >>> I need to think a bit more for a better solution.
> >>>
> >>> Best regards,
> >>> Liviu
> >>>
> >>>>
> >>>> Rob
> >>>>
> >>>
> >>
> >>
> >> -- 
> >> Thanks!
> >> Yijing
> >>
> >>
> > 
> 
> 
> -- 
> Thanks!
> Yijing
> 
> --
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