Date:	Fri, 22 Aug 2014 13:32:53 +0100
From:	Liviu Dudau <liviu@...au.co.uk>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
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>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH] drivers: pci: convert generic host controller to DT host
 bridge creation API

On Fri, Aug 22, 2014 at 12:13:33AM -0500, Bjorn Helgaas wrote:
> On Thu, Aug 21, 2014 at 6:01 PM, Liviu Dudau <liviu@...au.co.uk> wrote:
> > On Thu, Aug 21, 2014 at 12:02:16PM -0600, Bjorn Helgaas wrote:
> >> [+cc Lorenzo]
> >>
> >> On Wed, Aug 20, 2014 at 05:35:59PM -0500, Bjorn Helgaas wrote:
> >> > 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.
> >>
> >> I talked to Lorenzo here at LinuxCon and he explained this so it makes a
> >> lot more sense to me now.  Would something like the following work?
> >>
> >>   gen_pci_probe()
> >>   {
> >>     LIST_HEAD(res);
> >>     resource_size_t io_base = 0;
> >>
> >>     of_parse_pci_host_bridge_resources(dev, &res, 0, 0xff, &io_base);
> >>     gen_pci_setup(&res, io_base);
> >>
> >>     pci_create_root_bus(..., &res);
> >>     pci_scan_child_bus();
> >>     ... pci_assign_unassigned_bus_resources
> >>     pci_bus_add_resources();
> >>   }
> >>
> >> Then we at least have all the PCI-related code consolidated, without
> >> the arch-specific stuff mixed in.  We could almost use pci_scan_root_bus(),
> >> but not quite, because of the pci_assign_unassigned_bus_resources() call
> >> that pci_scan_root_bus() doesn't do.
> >
> > Hmm, after having a little bit more time to get my brain back into the problem
> > I'm now not sure this will be good enough.
> >
> > Let me explain what I was trying to solve with the callback that Arnd doesn't
> > like and maybe you (both) can validate if my concerns are real or not:
> >
> > I was trying to come up with a function that can easily replace pci_scan_child_bus().
> > The problem I'm facing is that the ranges that we parse from the device tree
> > need to be converted to resources and passed on to the pci_host_bridge structure
> > to be stored as windows. In order for the host bridge driver to be initialised, it
> > also needs to parse the device tree information *and* use the information calculated
> > for the io_base in order to program the address translation correctly. The only way
> > I found to avoid duplicating the parsing step and sequence the initialisation correctly
> > was to introduce the callback.
> 
> The current v9 patch has this:
> 
>   int of_create_pci_host_bridge(...)
>   {
>     of_pci_parse_bus_range();
>     pci_host_bridge_of_get_ranges();
>     pci_create_root_bus();
>     setup();
>     pci_scan_child_bus();
>     pci_assign_unassigned_bus_resources();
>     pci_bus_add_devices();
>   }
> 
> I don't think there's anything that requires setup() to be done after
> pci_create_root_bus().  You do pass the "struct pci_host_bridge *" to
> setup(), but I think that's only a convenient way to pass in the
> resource information that's also passed to pci_create_root_bus().
> setup() doesn't actually require the pci_bus or pci_host_bridge
> structures themselves.  So I think setup() *could* be done before
> pci_create_root_bus().  I think that would be better because then all
> the PCI core stuff is together and can be more easily factored out.
> 

OK. In my mind the setup part was not worth doing if we failed to create the root
bus, hence the order I've put them in. Also, up to v8 and in line with the
side discussions we had over the last year, the domain information and future
data was/could be stored in the pci_host_bridge structure, which again introduces
order. But I agree that in the current shape the setup can be done before
pci_create_root_bus().

> If setup() is before pci_create_root_bus(), my objective is met, and I
> don't care how you structure the rest.  Arnd prefers to avoid the
> callback, and I do agree that we should avoid callbacks when possible.
> 

I am slightly cagey that I have started with a grand plan of trying to create
a generic framework that can be future proof and now I'm trying to make the code
fit the existing API without too much insight on how it can be used in the future.
If you think this will be alright in your future plans of making pci_host_bridge
structure more central to the PCI framework, then I'm happy with it.

Best regards,
Liviu

> > If I follow your suggestion with gen_pci_probe() the question I have is about
> > gen_pci_setup(). Is that something that is implemented at architectural level?
> > If so, then we are droping into the arm implementation where each host bridge
> > driver has to register another hook to be called from arch code. If not, then
> > we risk having a platform with two host bridges, each implementing
> > gen_pci_setup().
> 
> The gen_pci_setup() in Lorenzo's patch [1] is already in
> pci-host-generic.c and doesn't look arch-specific to me.
> 
> [1] http://lkml.kernel.org/r/1407861695-25549-1-git-send-email-Liviu.Dudau@arm.com
> 
> > Arnd, one thing I'm trying to figure out is if you are not actually seeing the
> > callback I've introduced as a repeat of the arm implementation. That is not
> > my intent and it is designed to be used only by the host bridge drivers in order
> > to finish their initialisation once all the generic and architectural code has
> > run, but before any actual scanning of the root bus happens.
> >
> > Best regards,
> > Liviu
> >
> >>
> >> Bjorn
> >>
> >
> > --
> > -------------------
> >    .oooO
> >    (   )
> >     \ (  Oooo.
> >      \_) (   )
> >           ) /
> >          (_/
> >
> >  One small step
> >    for me ...
> >
> --
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> 

-- 
-------------------
   .oooO
   (   )
    \ (  Oooo.
     \_) (   )
          ) /
         (_/

 One small step
   for me ...

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