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

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Date:	Thu, 20 Nov 2014 13:53:48 +0100
From:	Tomasz Nowicki <tomasz.nowicki@...aro.org>
To:	Liviu Dudau <liviu@...au.co.uk>
CC:	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org,
	Tony Luck <tony.luck@...el.com>,
	Russell King <linux@....linux.org.uk>,
	linux-pci@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Xinwei Hu <huxinwei@...wei.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Suravee.Suthikulpanit@....com,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Yijing Wang <wangyijing@...wei.com>,
	linux-ia64@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Wuyun <wuyun.wu@...wei.com>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH 00/16] Refine PCI host bridge scan interfaces

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:
>>>> https://patchwork.ozlabs.org/patch/407585/
>>>>
>>>> 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 ?

>
> 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.

Tomasz

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