linux-kernel - Re: [PATCH v10 4/6] PCI: hisi: Add PCIe host support for HiSilicon SoC Hip05

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Date:	Tue, 13 Oct 2015 13:18:33 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Gabriele Paoloni <gabriele.paoloni@...wei.com>
Cc:	"Wangzhou (B)" <wangzhou1@...ilicon.com>,
	Bjorn Helgaas <helgaas@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"jingoohan1@...il.com" <jingoohan1@...il.com>,
	"pratyush.anand@...il.com" <pratyush.anand@...il.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"thomas.petazzoni@...e-electrons.com" 
	<thomas.petazzoni@...e-electrons.com>,
	"lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
	"james.morse@....com" <james.morse@....com>,
	"Liviu.Dudau@....com" <Liviu.Dudau@....com>,
	"jason@...edaemon.net" <jason@...edaemon.net>,
	"robh@...nel.org" <robh@...nel.org>,
	"gabriel.fernandez@...aro.org" <gabriel.fernandez@...aro.org>,
	"Minghuan.Lian@...escale.com" <Minghuan.Lian@...escale.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	zhangjukuo <zhangjukuo@...wei.com>,
	qiuzhenfa <qiuzhenfa@...ilicon.com>,
	"liudongdong (C)" <liudongdong3@...wei.com>,
	qiujiang <qiujiang@...wei.com>,
	"xuwei (O)" <xuwei5@...ilicon.com>,
	"Liguozhu (Kenneth)" <liguozhu@...ilicon.com>,
	"Wangkefeng (Kevin)" <wangkefeng.wang@...wei.com>,
	Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v10 4/6] PCI: hisi: Add PCIe host support for HiSilicon SoC Hip05

On Tuesday 13 October 2015 06:58:42 Gabriele Paoloni wrote:
> 
> > >> +
> > >> +static int __init hisi_pcie_init(void)
> > >> +{
> > >> +	return platform_driver_probe(&hisi_pcie_driver, hisi_pcie_probe);
> > >> +}
> > >> +subsys_initcall(hisi_pcie_init);
> > >
> > > Can you use module_platform_driver() or module_platform_driver_probe()
> > > here instead of the subsys_initcall()?  No, I don't really know what
> > > the difference between module_platform_driver() and
> > > module_platform_driver_probe() is, sorry :)

module_platform_driver_probe() will only call the probe function once
(and fail in case of -EPROBE_DEFER), while module_platform_driver()
installs the platform driver in a way that the device can be bound
and unbound at any point.

> > In fact, I used module_platform_driver_probe in previous version, but
> > A PCIe VGA card of HiSilicon will use Hip05 PCIe host, so we modified
> > module_platform_driver_probe to subsys_initcall which will be called
> > before module_platform_driver_probe.
> > 
> > We will upstream the driver of above PCIe VGA card soon.

I don't see a reason why a VGA driver would need the PCI host to be
probed this early, unless it is the only usable console in the system.

> Hi Bjorn, firstly many thanks for looking at this.
> 
> About this last bit the reason why we use subsys_initcall() is that
> our host bridge is embedded in the SoC and as such is not hot-pluggable
> for instance see:
> http://lxr.free-electrons.com/source/Documentation/driver-model/platform.txt#L59

We should still be able to build the driver as a loadable module,
even if you don't do that on your own kernels. 

This doesn't mean that it has to be module_platform_driver, subsys_initcall
will also work in a loadable module, it just won't be as early. However,
we should try to come up with a consistent approach for all PCI host drivers,
I don't see any reason for hisi to be different from the others here.

> Our approach is also used by Exynos:
> http://lxr.free-electrons.com/source/drivers/pci/host/pci-exynos.c#L644

Maybe we should change that too.

	Arnd
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