linux-kernel - Re: [PATCH]PCI:disable resource decode in PCI BAR detection

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Message-Id: <1189972912.6403.16.camel@localhost.localdomain>
Date:	Sun, 16 Sep 2007 22:01:52 +0200
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Ivan Kokshaysky <ink@...assic.park.msu.ru>
Cc:	Greg KH <gregkh@...e.de>, Matthew Wilcox <matthew@....cx>,
	Shaohua Li <shaohua.li@...el.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-pci <linux-pci@...ey.karlin.mff.cuni.cz>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH]PCI:disable resource decode in PCI BAR detection

On Thu, 2007-09-13 at 15:16 +0400, Ivan Kokshaysky wrote:
> On Thu, Sep 13, 2007 at 02:53:13AM -0700, Greg KH wrote:
> > On Thu, Sep 13, 2007 at 01:55:36AM -0600, Matthew Wilcox wrote:
> > > Unfortunately if this patch does cause any machine to break, these will
> > > be machines that worked fine up until this point, so that would be a
> > > regression, which is worse.  Life sucks.
> > 
> > If, after a while, you think the change should go into the -stable tree,
> > I have no objection.
> 
> I think it shouldn't - this change will almost certainly cause a regression.
> There is a lot of system devices besides the host bridges that shouldn't be
> disabled during BAR probe, like interrupt controllers, power management
> controllers and so on.
> 
> We need a more sophisticated fix - I'm thinking of introducing "probe" field
> in struct pci_dev which can be set by "early" quirk routines.

Agreed. I have a similar problem on ppc where it's common to have things
like the main PIC on a PCI device. Note that another problem is (or at
least was, i haven't checked recently) the P2P bridge scanning code
that, in a similar way, can block the path to all devices below it. I
-do- have a case for example with Apple Xserve G4's where the main Apple
IO ASIC, which is a PCI device containing the PIC, the power management
controller, and various low level system control IOs is behind a pair of
P2P bridges.

One solution for us (PPC) is to enforce those devices and bridges to be
described in the OF tree, and generalize a bit the code we have for some
64 bits machines, that synthetizes the pci_dev's from the OF nodes
rather than probing. But that's not going to help other archs.

In fact, that's a problem we also have with
pci_assign_unassigned_resources() which will happily move things around
that must not be moved, especially when sitting behind P2P bridges.

So the root of the issue is much deeper than just a quirk here I
believe.

Cheers,
Ben.


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