linux-kernel - Re: [PATCH] drivers: pci: convert generic host controller to DT host bridge creation API

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Message-ID: <CAErSpo6Z4b3w7j=X9C1YGOV59JHZwZUt7pnVp+Z0erY2RLCNAQ@mail.gmail.com>
Date:	Wed, 20 Aug 2014 17:35:59 -0500
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Liviu Dudau <liviu@...au.co.uk>
Cc:	Arnd Bergmann <arnd@...db.de>, Liviu Dudau <Liviu.Dudau@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	Jingoo Han <jg1.han@...sung.com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Suravee Suthikulanit <suravee.suthikulpanit@....com>,
	linux-pci <linux-pci@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	LAKML <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] drivers: pci: convert generic host controller to DT host
 bridge creation API

On Wed, Aug 20, 2014 at 7:31 AM, Liviu Dudau <liviu@...au.co.uk> wrote:
> On Wed, Aug 20, 2014 at 01:27:57PM +0200, Arnd Bergmann wrote:
>> On Tuesday 12 August 2014, Liviu Dudau wrote:
>> > +       return of_create_pci_host_bridge(dev, 0, 0xff, &gen_pci_ops,
>> > +                                       gen_pci_setup, pci);
>>
>> I had not noticed it earlier, but the setup callback is actually a feature
>> of the arm32 PCI code that I had hoped to avoid when moving to the
>> generic API. Can we do this as a more regular sequence of
>>
>>
>>       ret = of_create_pci_host_bridge(dev, 0, 0xff, &gen_pci_ops, pci);
>>       if (ret)
>>               return ret;
>>
>>       ret = gen_pci_setup(pci);
>>       if (ret)
>>               pci_destroy_host_bridge(dev, pci);
>>       return ret;
>>
>> ?
>>
>>       Arnd
>
> Hi Arnd,
>
> That has been the general approach of my patchset up to v9. But, as Bjorn has
> mentioned in his v8 review and I have put in my cover letter, the regular
> aproach means that architectures that use pci_scan_root_bus() will have to
> drop their one liner and replace it with the more verbose of_create_pci_host_bridge()
> followed by pci_scan_child_bus() and pci_bus_add_devices() (basically, the content
> of pci_scan_root_bus()). For those architectures it will lead to a net increase of
> lines of code.
>
> The patch for pci-host-generic.c is the first to use the callback setup function, but
> not the only one. My PCI host bridge driver for Juno has the same need, and I'm betting
> all other host bridge controllers will use it as it will be the only opportunity to
> finish the controller setup before we start scanning the child busses. I'm trying to
> balance ease of read vs ease of use here and it is the best version I've come up with
> so far.

My guess is that you're referring to
http://lkml.kernel.org/r/20140708011136.GE22939@google.com

I'm trying to get to the point where arch code can discover the host
bridge, configure it, learn its properties (apertures, etc.), then
pass it off completely to the PCI core for PCI device enumeration.
pci_scan_root_bus() is the closest thing we have to that right now, so
that's why I point to that.  Here's the current pci_scan_root_bus():

  pci_scan_root_bus()
  {
    pci_create_root_bus();
    /* 1 */
    pci_scan_child_bus()
    /* 2 */
    pci_bus_add_devices()
  }

This is obviously incomplete as it is -- for example, it does nothing
about assigning resources to PCI devices, so it only works if we rely
completely on the firmware to do that.  Some arches (x86, ia64, etc.)
don't want to rely on firmware, so they basically open-code
pci_scan_root_bus() and insert resource assignment at (2) above.  That
resource assignment really *should* be done in pci_scan_root_bus()
itself, but it's quite a bit of work to make that happen.

In your case, of_create_pci_host_bridge() open-codes
pci_scan_root_bus() and calls the "setup" callback at (1) in the
outline above.  I don't have any problem with that, and I don't care
whether you do it by passing in a callback function pointer or via
some other means.

However, I would ask whether this is really a requirement.  Most
(maybe all) other arches require nothing special at (1), i.e., between
pci_create_root_bus() and pci_scan_child_bus().  If you can do it
*before* pci_create_root_bus(), I think that would be nicer, but maybe
you can't.

Bjorn
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