linux-kernel - Re: [PATCH -v11 04/30] PCI: Add busn_res into struct pci

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Message-ID: <CAErSpo5EdBY-8M5FnQaGHOAvhaGO7qLHdDkmYGyDRN9de4umNw@mail.gmail.com>
Date:	Tue, 1 May 2012 09:40:05 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Tony Luck <tony.luck@...el.com>,
	David Miller <davem@...emloft.net>, x86 <x86@...nel.org>,
	Dominik Brodowski <linux@...inikbrodowski.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH -v11 04/30] PCI: Add busn_res into struct pci_bus.

On Tue, May 1, 2012 at 12:27 AM, Yinghai Lu <yinghai@...nel.org> wrote:
> On Mon, Apr 30, 2012 at 9:02 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
>> On Mon, Apr 30, 2012 at 6:32 PM, Yinghai Lu <yinghai@...nel.org> wrote:
>>> On Mon, Apr 30, 2012 at 4:25 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
>>>>> --- a/include/linux/pci.h
>>>>> +++ b/include/linux/pci.h
>>>>> @@ -419,6 +419,7 @@ struct pci_bus {
>>>>>        struct list_head slots;         /* list of slots on this bus */
>>>>>        struct resource *resource[PCI_BRIDGE_RESOURCE_NUM];
>>>>>        struct list_head resources;     /* address space routed to this bus */
>>>>> +       struct resource busn_res;       /* track registered bus num range */
>>>>>
>>>>>        struct pci_ops  *ops;           /* configuration access functions */
>>>>>        void            *sysdata;       /* hook for sys-specific extension */
>>>>
>>>> struct pci_bus already includes "secondary" and "subordinate".    This
>>>> new "busn_res" looks like it will contain the same information.  Why
>>>> do we need both?
>>>
>>> In some case the could be different.
>>> for root bus from _CRS, busn_res could bigger than subordinate,
>>> because scan_childbus will update subordinate.
>>
>> For a bus below a P2P bridge, I think it's always the case that the
>> bridge's Subordinate Bus Number in config space == bus->subordinate ==
>> bus->busn_res.end (correct me if I'm wrong).  I don't like the
>> redundancy in this case.
>
> there are about 70 bus->subordinate reference and 40 bus->secondary reference.
>
> could try to update them in following patch set.

If you're proposing this:
  1. add bus->busn_res
  2. remove bus->subordinate and bus->secondary
I fully support that, and I'd like to merge both pieces at the same
time (different patches is fine; I just want to make sure both pieces
actually happen).

>> For a root bus where you set bus->busn_res from _CRS and
>> bus->subordinate = pci_scan_child_bus(), bus->busn_res.end will
>> generally be different from bus->subordinate, but there's no point in
>> keeping track of bus->subordinate.
>>
>> The reason we care about secondary and subordinate is so we can
>> allocate bus numbers when enumerating devices behind a bridge.  The
>> only thing we need for that is the aperture of the upstream bridge and
>> the apertures of any peer bridges on the same bus.  Let's say we have
>> this:
>>
>>        pci 00:00.0 bridge to [bus a-b]
>>        pci a:01.0 bridge to [bus c-d]  (already enumerated)
>>        pci a:02.0 bridge to [bus e-f]  (already enumerated)
>>        pci a:03.0 bridge to [bus x-y]  (enumerating now)
>>
>> We know [c-d] is contained in [a-b]; [e-f] is contained in [a-b]; a <
>> c; and a < e.  To enumerate behind a:03.0, we need to assign x & y
>> such that a < x; [x-y] is contained in [a-b]; and [x-y] does not
>> overlap [c-d] or [e-f].  The value from pci_scan_child_bus() is
>> probably useful for setting y, but we don't have to save it in the
>> struct pci_bus for that.
>
> busn alloc will try to solve x-y may need big range than [a,b], it
> will extend top of b and parents of bus a.
> instead of just b+1 blindly.
>
> and will have more strict checking to avoid overlapping.

Obviously the completely general problem of allocating bus numbers may
require traversing up the tree.  My point is that I don't think it's
necessary to keep both busn_res.end and subordinate to do that.

>>> and also we have one resource to insert it into the resource tree, so
>>> later could probe/allocate bus num range.
>>
>> Sorry, I didn't understand this.
>
> Using busn_res to track and allocate busn range, by put them in the
> resource tree could reuse resource allocating code.

Yes, I agree that replacing secondary & subordinate with a struct
resource is a good idea.  That will allow a resource tree of bus
numbers, as well as other useful things like the ability to "%pR".

I just don't want *both* busn_resource and secondary & subordinate.

Bjorn
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