linux-kernel - Re: [patch] pci: revert "PCI: remove transparent bridge sizing"

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Message-Id: <1206570448.6926.27.camel@pasglop>
Date:	Thu, 27 Mar 2008 09:27:28 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Gary Hade <garyhade@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Meyer <thomas@...3r.de>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>,
	Adrian Bunk <bunk@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Natalie Protasevich <protasnb@...il.com>, pm@...ian.org
Subject: Re: [patch] pci: revert "PCI: remove transparent bridge sizing"


On Wed, 2008-03-26 at 22:07 +0000, Alan Cox wrote:
> > PCI bridges at zero is perfectly valid indeed and I'm sure we have that
> > around at least for IO space. In fact, I'm surprised you don't have that
> > on x86. Typically, things like an HT segment with a P2P bridge and
> > behind that bridge an ISA bridge could well have the P2P bridge with a
> > resource forwarding 0...0x1000 IO downstream for example even on x86
> > no ? (I'm not -that- familiar with the crazyness of legacy ISA on x86
> > but I've definitely seen such setup on other archs).
> 
> On a PC system 0x00-0xFF are motherboard resources (sometimes chipset,
> some even swallowed by the CPU in certain cases) so 0 as disabled is sort
> of safe but as shown by the pci_enable_device_bars replacement code - not
> a good idea neccessarily.
> 
> A lot of driver code does assume 0 == unavailable/off/disabled including
> large chunks of serial, ata, ide and probably other subsystems.

True, and we do try to avoid allocating resources at 0 most of the time,
but it happens, and in the case of bridges, it can make somewhat sense
(more in fact than for devices I'd say).

Cheers,
Ben.


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