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

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Date:	Mon, 30 Apr 2012 23:27:58 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
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 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.

>
> 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.

>
>> 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.

Yinghai
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