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

[Jayachandran C]

On Mon, May 2, 2016 at 6:13 PM, Tomasz Nowicki <tn@xxxxxxxxxxxx> wrote:
> 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.

In this case, getting the domain should be trivial since the ACPI
companion on parent is already set, this should work

int acpi_pci_bus_domain_nr(struct device *parent)
{
        struct acpi_device *acpi_dev = to_acpi_device(parent);
        struct acpi_pci_root *root = acpi_dev->driver_data;

        return root->segment;
}

Or am I missing something here?

JC.