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

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Date:	Tue, 18 Sep 2007 06:30:23 +1000
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 Mon, 2007-09-17 at 14:22 +0400, Ivan Kokshaysky wrote:
> On Sun, Sep 16, 2007 at 10:01:52PM +0200, Benjamin Herrenschmidt wrote:
> > 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.
> 
> I think the P2P probing code is pretty safe now - there are read-only
> accesses to the bridge config, unless you request to reassign the bus
> numbers. Though it won't be safe anymore with the patch in question.

In which case I will need to NAK the patch... Note that those Xserve
G4's still have the subtle issue that they -also- reassign bus
numbers :-) But that's going away the day I finally enable domains
support for ppc32 (it's been off for now due to problems with X)

> > 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.
> 
> If you can get reliable PCI info from firmware, it should be relatively easy
> to avoid at least a bar sizing. You can install an "early" fixup for
> PCI_ANY_ID and fill the resource fields of the pci_dev with values obtained
> from firmware. Then all we need in probe.c is just to check that the resource
> is already non-zero and skip the sizing of respective BAR, if so.

Right now, we have code to completely build a pci_dev from the firmware
infos. We only use it on 64 bits pseries currently though. 

> > 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.
> 
> It's not supposed to do that. Certainly, there were problems of that sort,
> but hopefully they are in the past.

At this stage (but we are getting a bit OT), ppc has something like 3
different PCI code implementations :-) I do have some plans to fix that
by switching everybody to use pci_assign_unassigned_resources() and
friends but last time I tried, everything blew up :-) I suspect I'll
need a quirk or two in the generic code, but I'll let you know when I
get to it.

Cheers,
Ben.

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