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

[Yijing Wang]

>>> 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.

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