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

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Date:	Wed, 14 Oct 2015 09:31:48 +0000
From:	Gabriele Paoloni <gabriele.paoloni@...wei.com>
To:	Arnd Bergmann <arnd@...db.de>
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



> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@...db.de]
> Sent: Wednesday, October 14, 2015 10:04 AM
> To: Gabriele Paoloni
> Cc: Wangzhou (B); Bjorn Helgaas; Bjorn Helgaas; jingoohan1@...il.com;
> pratyush.anand@...il.com; linux@....linux.org.uk;
> thomas.petazzoni@...e-electrons.com; lorenzo.pieralisi@....com;
> james.morse@....com; Liviu.Dudau@....com; jason@...edaemon.net;
> robh@...nel.org; gabriel.fernandez@...aro.org;
> Minghuan.Lian@...escale.com; linux-pci@...r.kernel.org; linux-arm-
> kernel@...ts.infradead.org; devicetree@...r.kernel.org; linux-
> kernel@...r.kernel.org; zhangjukuo; qiuzhenfa; liudongdong (C);
> qiujiang; xuwei (O); Liguozhu (Kenneth); Wangkefeng (Kevin); Rob
> Herring
> Subject: Re: [PATCH v10 4/6] PCI: hisi: Add PCIe host support for
> HiSilicon SoC Hip05
> 
> On Wednesday 14 October 2015 08:34:43 Gabriele Paoloni wrote:
> > > -----Original Message-----
> > > From: Arnd Bergmann [mailto:arnd@...db.de]
> > > Sent: Tuesday, October 13, 2015 12:19 PM
> > > To: Gabriele Paoloni
> > > Cc: Wangzhou (B); Bjorn Helgaas; Bjorn Helgaas;
> jingoohan1@...il.com;
> > > pratyush.anand@...il.com; linux@....linux.org.uk;
> > > thomas.petazzoni@...e-electrons.com; lorenzo.pieralisi@....com;
> > > james.morse@....com; Liviu.Dudau@....com; jason@...edaemon.net;
> > > robh@...nel.org; gabriel.fernandez@...aro.org;
> > > Minghuan.Lian@...escale.com; linux-pci@...r.kernel.org; linux-arm-
> > > kernel@...ts.infradead.org; devicetree@...r.kernel.org; linux-
> > > kernel@...r.kernel.org; zhangjukuo; qiuzhenfa; liudongdong (C);
> > > qiujiang; xuwei (O); Liguozhu (Kenneth); Wangkefeng (Kevin); Rob
> > > Herring
> > > 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.
> >
> > Hi Arnd, I don't see the point of having loadable KOs for platform
> > devices that are integrated into SoCs (like PCIe Host Controllers...)
> 
> Mainly we want as many drivers as possible to be loadable modules,
> and there is no reason why PCI needs to be different from other
> subsystems here.

Ok I see now. Thanks

> 
> > > 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.
> >
> > To me it sounds more appropriate to adopt subsys_initcall() for all
> the
> > PCI Host Bridge controllers rather than having them as loadable
> modules...
> >
> > What is your view?
> 
> subsys_initcall() sounds odd because it's a driver rather than a
> subsystem,
> but I realize that most of the other levels don't fit any better.

Yes well I was seeing for example the vgaarb 
http://lxr.free-electrons.com/source/drivers/gpu/vga/vgaarb.c#L1357

That in the init is calling pci_get_subsys()

So I was wondering that the PCI devices may not be registered unless
we also init the PCI host bridge through subsys_initcall()...

But then maybe is the vgaarb to be buggy...

> 
> As I said, it's not really a choice we have to make in the source code,
> we can use subsys_initcall together with module_exit(), or we can
> create a helper macro that is similar to module_platform_driver()
> specifically for PCI that uses a particular initcall level.

Ok got it. But I guess this needs to be thought and applied to all
the PCI host bridge controllers...

So maybe for this driver I can use module_platform_driver_probe()
and then we can see...

> 
> 	Arnd
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