linux-kernel - Re: Linux 2.6.27-rc5: System boot regression caused by commit a2bd7274b47124d2fc4dfdb8c0591f545ba749dd

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Date:	Sat, 30 Aug 2008 21:20:09 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeff Garzik <jeff@...zik.org>, Tejun Heo <htejun@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	David Witbrodt <dawitbro@...global.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kernel Testers <kernel-testers@...r.kernel.org>
Subject: Re: Linux 2.6.27-rc5: System boot regression caused by commit a2bd7274b47124d2fc4dfdb8c0591f545ba749dd

On Saturday, 30 of August 2008, Linus Torvalds wrote:
> 
> On Sat, 30 Aug 2008, Rafael J. Wysocki wrote:
> > 
> > > And if you have the whole dmesg, that would be useful.
> > 
> > dmesg from -rc5 with the offending commit reverted and with the patch
> > below applied is at:
> > 
> > http://www.sisk.pl/kernel/debug/mainline/2.6.27-rc5/2.6.27-rc5-git.log
> 
> Ok, the more I look at this, the more interesting it gets.
> 
> In particular, this:
> 
> 	...
> 	ACPI: bus type pnp registered
> 	pnp 00:08: mem resource (0xfec00000-0xfec00fff) overlaps 0000:00:00.0 BAR 3 (0xe0000000-0xffffffff), disabling
> 	pnp 00:08: mem resource (0xfee00000-0xfee00fff) overlaps 0000:00:00.0 BAR 3 (0xe0000000-0xffffffff), disabling
> 	pnp 00:09: mem resource (0xffb80000-0xffbfffff) overlaps 0000:00:00.0 BAR 3 (0xe0000000-0xffffffff), disabling
> 	pnp 00:09: mem resource (0xfff00000-0xffffffff) overlaps 0000:00:00.0 BAR 3 (0xe0000000-0xffffffff), disabling
> 	pnp 00:0b: mem resource (0xe0000000-0xefffffff) overlaps 0000:00:00.0 BAR 3 (0xe0000000-0xffffffff), disabling
> 	pnp 00:0c: mem resource (0xfec00000-0xffffffff) overlaps 0000:00:00.0 BAR 3 (0xe0000000-0xffffffff), disabling
> 	pnp: PnP ACPI: found 13 devices
> 	ACPI: ACPI bus type pnp unregistered
> 	SCSI subsystem initialized
> 	libata version 3.00 loaded.
> 	usbcore: registered new interface driver usbfs
> 	usbcore: registered new interface driver hub
> 	usbcore: registered new device driver usb
> 	PCI: Using ACPI for IRQ routing
> 	pci 0000:00:00.0: BAR 3: can't allocate resource
> 	...
> 
> there's a few things to note here:
> 
>  - the resource at 0000:00:00.0 BAR 3 is totally bogus.
> 
>    We know it's totally bogus because you actually have other resources in 
>    the 0xf....... range, and they work fine. It's also likely to be 
>    totally bogus because it so happens that the end-point of 0xffffffff is 
>    commonly something that the BIOS leaves as a "I sized this resource", 
>    because that's how resources are sized (you write all ones into them 
>    and look what you can read back).
> 
>    But your lspci -vxx output clearly shows that (a) MEM is enabled in 
>    the command word, and yes, the BAR register at 0x18 does indeed have 
>    value 0xe0000000. So it's just the length that is really bogus.
> 
>  - pnp clearly sees that bogus resource at 0xe0000000-0xffffffff
> 
>  - BUT: the "can't allocate resource" thing is from 
>    pcibios_allocate_resources(), and means that the request_resource() 
>    failed _despite_ the fact that you hadn't reserved the e820 resources 
>    yet with the new patch.
> 
> The thing that seems to save you is that we've already allocated something 
> in that region. There's a few things there, like:
> 
> 	fee00000-fee00fff : Local APIC
> 
> but that particular one is actually reserved much later, so that doesn't 
> explain it. I think that what happens is that we have allocated the _bus_ 
> resources earlier in "pcibios_allocate_bus_resources()", and that means 
> that we already have these resources:
> 
> 	fe700000-fe7fffff : PCI Bus 0000:01
> 	fe800000-fe8fffff : PCI Bus 0000:02
> 	fe900000-fe9fffff : PCI Bus 0000:03
> 	fea00000-feafffff : PCI Bus 0000:04
> 	feb00000-febfffff : PCI Bus 0000:05
> 
> in the resource tree, and that in turn means that when we try to allocate 
> the bogus MCFG resource, it fails.
> 
> Which is good - it mustn't succeed.
> 
> What _broke_ for you is that the horrible patch that got reverted said 
> that "if we recognize this as an MCFG resource, we will _always_ try to 
> insert it", so it fundamentally broke the whole resource tree, because it 
> force-inserted that totally crap resource.

Well, I thought something like this happened, but I wasn't quite sure about the
exact mechanism.  Thanks for the explanation. :-)

Rafael
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