linux-kernel - Re: [PATCH 09/24] PCI, powerpc: Register busn

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Date:	Fri, 10 Feb 2012 08:35:58 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.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 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.

Cheers,
Ben.

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