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

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Date:	Tue, 18 Nov 2014 13:27:45 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Yijing Wang <wangyijing@...wei.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Liviu Dudau <liviu@...au.co.uk>,
	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>,
	Thierry Reding <thierry.reding@...il.com>,
	Suravee.Suthikulpanit@....com,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	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 Tuesday 18 November 2014 20:17:57 Yijing Wang wrote:
> 
> >>
> >> I hope platforms with ACPI or DT could both use pci_create_host_bridge().
> >> Why we need to use two different ways to process it ?
> > 
> > These are completely different use cases:
> > 
> > a) For DT, we want loadable device drivers that start by probing a host
> >    bridge device which was added through the DT platform code. The
> >    driver is self-contained, and eventually we want to be able to unload
> >    it. We have lots of different per-soc drivers that require different
> >    quirks
> > 
> > b) For ACPI, the interface is defined in the ACPI spec across architectures
> >    and SoCs, we don't have host bridge drivers and the code that initializes
> >    the PCI is required early during boot and called from architecture
> >    code. There is no parent device, as ACPI sees PCI as a fundamental building
> >    block by itself, and there are no drivers because the firmware does
> >    the initial hardware setup, so we only have to access the config space.
> 
> Hmmm, I'm a little confused, so why you think ACPI host driver should not use
> pci_create_host_bridge(), because ACPI PCI driver has no parent device ?

It's one of the difference. Having a parent device can certainly make your
life simpler, since you have devm_kzalloc(), dev_info(), etc. Coming from
the other end, I think ACPI needs PCI to be available during early boot,
at a time where we might not want pci_create_host_bridge() to do the
right thing.

	Arnd
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