Re: [PATCH V6 02/13] pci, acpi: Provide generic way to assign bus domain number.

From: Tomasz Nowicki
Date: Mon May 02 2016 - 08:44:12 EST

On 04/27/2016 01:17 PM, Lorenzo Pieralisi wrote:
On Tue, Apr 26, 2016 at 09:26:49PM -0500, Bjorn Helgaas wrote:
On Fri, Apr 15, 2016 at 07:06:37PM +0200, Tomasz Nowicki wrote:
+int acpi_pci_bus_domain_nr(struct device *parent)
+ struct acpi_device *acpi_dev = to_acpi_device(parent);
+ unsigned long long segment = 0;
+ acpi_status status;
+ /*
+ * If _SEG method does not exist, following ACPI spec (6.5.6)
+ * all PCI buses belong to domain 0.
+ */
+ status = acpi_evaluate_integer(acpi_dev->handle, METHOD_NAME__SEG, NULL,
+ &segment);
We already have code in acpi_pci_root_add() to evaluate _SEG. We
don't want to evaluate it *twice*, do we?

I was sort of expecting that if you added it here, we'd remove the
existing call, but it looks like you're keeping both?
We can't remove the existing call, since it is used on X86 and IA64
to store the segment number that, in the process, is used in their
pci_domain_nr() arch specific callback to retrieve the domain nr.

On ARM64, that selects PCI_DOMAINS_GENERIC, we have to find a way
to retrieve the domain number that is not arch dependent, since
this is generic code, we can't rely on any bus->sysdata format (unless
we do something like JC did below), therefore the only way is to call
the _SEG method *again* here, which also forced Tomasz to go through
the ACPI_COMPANION setting song and dance and pass the parent pointer
to pci_create_root_bus() (see patch 1), which BTW is a source of
trouble on its own as you noticed.
What trouble in patch 1 do you mean? I may miss something.

I agree that patch 1 is not necessary if we decide to use sysdata or rework root bus scanning to move domain to host bridge. Nevertheless, patch 1 is still a cleanup IMO.