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Date:	Thu, 23 Feb 2012 12:25:36 -0800
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Tony Luck <tony.luck@...el.com>,
	Dominik Brodowski <linux@...inikbrodowski.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 09/24] PCI, powerpc: Register busn_res for root buses

On Fri, 10 Feb 2012 08:35:58 +1100
Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:

> On Thu, 2012-02-09 at 11:24 -0800, Bjorn Helgaas wrote:
> > My point is that the interface between the arch and the PCI core
> > should be simply the arch telling the core "this is the range of bus
> > numbers you can use."  If the firmware doesn't give you the HW limits,
> > that's the arch's problem.  If you want to assume 0..255 are
> > available, again, that's the arch's decision.
> > 
> > But the answer to the question "what bus numbers are available to me"
> > depends only on the host bridge HW configuration.  It does not depend
> > on what pci_scan_child_bus() found.  Therefore, I think we can come up
> > with a design where pci_bus_update_busn_res_end() is unnecessary.
> 
> In an ideal world yes. In a world where there are reverse engineered
> platforms on which we aren't 100% sure how thing actually work under the
> hood and have the code just adapt on "what's there" (and try to fix it
> up -sometimes-), thinks can get a bit murky :-)
> 
> But yes, I see your point. As for what is the "correct" setting that
> needs to be done so that the patch doesn't end up a regression for us,
> I'll have to dig into some ancient HW to dbl check a few things. I hope
> 0...255 will just work but I can't guarantee it.
> 
> What I'll probably do is constraint the core to the values in
> hose->min/max, and update selected platforms to put 0..255 in there when
> I know for sure they can cope.

But I think the point is, can't we intiialize the busn resource after
the first & last bus numbers have been determined?  E.g. rather than
Yinghai's current:
+	pci_bus_insert_busn_res(bus, hose->first_busno, hose->last_busno);
+
 	/* Get probe mode and perform scan */
 	mode = PCI_PROBE_NORMAL;
 	if (node && ppc_md.pci_probe_mode)
@@ -1742,8 +1744,11 @@ void __devinit pcibios_scan_phb(struct pci_controller *hose)
 		of_scan_bus(node, bus);
 	}
 
-	if (mode == PCI_PROBE_NORMAL)
+	if (mode == PCI_PROBE_NORMAL) {
+		pci_bus_update_busn_res_end(bus, 255);
 		hose->last_busno = bus->subordinate = pci_scan_child_bus(bus);
+		pci_bus_update_busn_res_end(bus, bus->subordinate);
+	}

we'd have something more like:

 	/* Get probe mode and perform scan */
 	mode = PCI_PROBE_NORMAL;
 	if (node && ppc_md.pci_probe_mode)
@@ -1742,8 +1744,11 @@ void __devinit pcibios_scan_phb(struct pci_controller *hose)
 		of_scan_bus(node, bus);
 	}
 
	if (mode == PCI_PROBE_NORMAL)
 		hose->last_busno = bus->subordinate = pci_scan_child_bus(bus);

+	pci_bus_insert_busn_res(bus, hose->first_busno, hose->last_busno);

since we should have the final bus range by then?  Setting the end to
255 and then changing it again doesn't make sense; and definitely makes
the code hard to follow.

-- 
Jesse Barnes, Intel Open Source Technology Center

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