Re: [RFC PATCH 00/16] Refine PCI host bridge scan interfaces

From: Yijing Wang
Date: Thu Nov 20 2014 - 21:59:13 EST

On 2014/11/21 0:39, Liviu Dudau wrote:
> On Thu, Nov 20, 2014 at 01:53:48PM +0100, Tomasz Nowicki wrote:
>> On 20.11.2014 13:08, Liviu Dudau wrote:
>>> On Thu, Nov 20, 2014 at 12:54:48PM +0100, Tomasz Nowicki wrote:
>>>> On 17.11.2014 15:13, Arnd Bergmann wrote:
>>>>> On Monday 17 November 2014 18:21:34 Yijing Wang wrote:
>>>>>> This series is based Linux 3.18-rc1 and Lorenzo Pieralisi's
>>>>>> arm PCI domain cleanup patches, link:
>>>>>> Current pci scan interfaces like pci_scan_root_bus() and directly
>>>>>> call pci_create_root_bus()/pci_scan_child_bus() lack flexiblity.
>>>>>> Some platform infos like PCI domain and msi_chip have to be
>>>>>> associated to PCI bus by some arch specific function.
>>>>>> We want to make a generic pci_host_bridge, and make it hold
>>>>>> the platform infos or hook. Then we could eliminate the lots
>>>>>> of arch pci_domain_nr, also we could associate some platform
>>>>>> ops something like pci_get_msi_chip(struct pci_dev *dev)
>>>>>> with pci_host_bridge to avoid introduce arch weak functions.
>>>>>> This RFC version not for all platforms, just applied the new
>>>>>> scan interface in x86/arm/powerpc/ia64, I will refresh other
>>>>>> platforms after the core pci scan interfaces are ok.
>>>>> I think overall this is a good direction to take, in particular
>>>>> moving more things into struct pci_host_bridge so we can
>>>>> slim down the architecture specific code.
>>>>> I don't particularly like the way you use the 'pci_host_info'
>>>>> to pass callback pointers and some of the generic information.
>>>>> This duplicates some of the issues we are currently trying
>>>>> to untangle in the arm32 code to make drivers easier to share
>>>>> between architectures.
>>>>> As a general approach, I'd rather see generic helper functions
>>>>> being exported by the PCI core that a driver may or may not
>>>>> call.
>>>>> The way you split the interface between things that happen
>>>>> before scanning the buses (pci_create_host_bridge) and
>>>>> the actual scanning (__pci_create_root_bus, pci_scan_child_bus)
>>>>> seems very helpful and I think we can expand that concept further:
>>>>> - The normal pci_create_host_bridge() function can contain
>>>>> all of the DT scanning functions (finding bus/mem/io resources,
>>>>> finding the msi-parent), while drivers that don't depend on DT
>>>>> for this information can call the same function and fill the
>>>>> same things after they have the pci_host_bridge pointer.
>>>> How about finding PCI domain number (in the DT way) within
>>>> pci_create_host_bridge() too ?
>>> It is an idea worth pursuing for the 99% of the cases. I would like
>>> to understand the 1% of the time when we want a domain number to be
>>> shared between two host bridges or the time when we want more than
>>> one domain per bridge.
>> Even though we have shared domain, this should be resolved via DT calls, do
>> I miss something ?
> If we only going to hold one domain number per host bridge, then no, you're
> not missing anything.
>>> Is that possible? Is it useful? Is it already in practice?
>> This is good question... IMO:
>> 1. Two host bridges can shared domain number if they are children of the
>> same parent host bridge.
>> 2. But I can not find good explanation for more than one domain per bridge.
> Splitting a root bus into two or more "segments" ?

It seems impossible.

> Best regards,
> Liviu
>> Tomasz


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