linux-kernel - Re: [PATCH 08/18] PCI, powerpc: Register busn

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Date:	Tue, 28 Feb 2012 19:54:32 +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>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.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 08/18] PCI, powerpc: Register busn_res for root buses

On Mon, 2012-02-27 at 22:36 -0700, Bjorn Helgaas wrote:
> 
> There's a lot of powerpc code that does this:
> 
>     bus_range = of_get_property(pcictrl, "bus-range", &len);
>     hose->first_busno = bus_range[0];
>     hose->last_busno = bus_range[1];
> 
> That *looks* like it is discovering the bus number aperture.  Is it?
> If it is, why are we using the largest bus number found by
> pci_scan_child_bus() rather than "last_busno"?

We do that but we somewhat -also- rely on the core bumping it if it
needs to make room :-)

As I said, we are swimming in dirty waters between reverse engineered
stuff we don't know 100% and "designed" stuff.

I think we should have ways to more explicitely define what we want tho,
ie whether hose->last_busno is just what happens to be the "current" bus
number assigned by the firmware or the hard max. Maybe a pci flag ?

On the other hand some platforms (all the ppc4xx ones for example) set
the flag to reassign all busses ... but have limit on bus numbers simply
because they have a memory mapped only config space and we don't have
enough address space to ioremap it all on 32-bit.

We need to fix them to use a fixmap entry to do atomic on-demand mapping
of the config space and lift that restriction, but that isn't done yet.

So I think those patches will need really careful handling on our side.

Cheers,
Ben.

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