pci: Omit error message for benign allocation failure

Hi,

On a system with PCI-to-PCI bridges, following errors are observed:

PCI: Failed to allocate mem resource #8:100000@d8200000 for 0000:02:00.0
PCI: Failed to allocate mem resource #6:10000@0 for 0000:03:01.0

'#6' is for expansion ROM and '#8' for the bridge where the device with the expansion ROM is connected.
But I think the failure is benign because the allocation is not necessary for these resources.

The allocation failure message is informative when the failure is really a problem and needs a diagnosis.
However, if the resource is expansion ROMs, the message is just confusing.

This patch omits the error message if the resource is an expansion ROM or a bridge.

Thanks,

Jun'ichi Nomura, NEC Corporation of America


Signed-off-by: Jun'ichi Nomura <[email protected]>

--- linux-2.6.24-rc3/drivers/pci/setup-res.c.orig	2007-12-04 00:24:11.000000000 -0500
+++ linux-2.6.24-rc3/drivers/pci/setup-res.c	2007-12-04 00:42:14.000000000 -0500
@@ -158,11 +158,12 @@ int pci_assign_resource(struct pci_dev *
 	}
 
 	if (ret) {
-		printk(KERN_ERR "PCI: Failed to allocate %s resource "
-			"#%d:%llx@%llx for %s\n",
-			res->flags & IORESOURCE_IO ? "I/O" : "mem",
-			resno, (unsigned long long)size,
-			(unsigned long long)res->start, pci_name(dev));
+		if (resno < PCI_ROM_RESOURCE)
+			printk(KERN_ERR "PCI: Failed to allocate %s resource "
+				"#%d:%llx@%llx for %s\n",
+				res->flags & IORESOURCE_IO ? "I/O" : "mem",
+				resno, (unsigned long long)size,
+				(unsigned long long)res->start, pci_name(dev));
 	} else if (resno < PCI_BRIDGE_RESOURCES) {
 		pci_update_resource(dev, res, resno);
 	}

I believe there is a good chance that may be another instance of the regression caused by my "Avoid creating P2P prefetch window for expansion ROMs" patch that was recently reported by Jan Beulich.
http://marc.info/?l=linux-kernel&m=119555581103023&w=2

You might want to try reverting my changes to see if the problem disappears.

I am working on a better fix for the problem that the patch was attempting to address but this is turning out to be much more difficult than I expected. If I don't have a solution very soon I plan to publish a revert patch.

This is an interesting idea. Could you elaborate? As far as I can tell, the kernel always tries to allocate memory for expansion ROMs which it also exports to user level. I have assumed that some drivers or user level apps may need to access this space. Is this not true?

If this is due to my change, sorry for the trouble.

Thanks,
Gary

Gary Hade
System x Enablement
IBM Linux Technology Center
503-578-4503  IBM T/L: 775-4503
[email protected]
http://www.ibm.com/linux/ltc

Hi Gary,

Sorry, I was not clear about that.
I've seen the above errors even with 2.6.22 and RHEL5, which is based on 2.6.18.
So it's not a regression.

Kernel always tries to. But it's best effort basis, IMO. (Maybe your patch is going to fix that?)
In the 1st stage (pcibios_allocate_resources, etc.),
it allocates resources based on PCI configuration provided by BIOS, except for expansion ROMs.
In the 2nd stage (pci_assign_unassigned_resources, etc.), it allocates resources for unallocated ones including expansion ROMs. The 2nd stage doesn't reprogram the bridge settings.
So if the expansion ROM is under the bridge where other resource is already allocated, the allocation failure occurs.
There are comments in drivers/pci/setup-bus.c:
/* Helper function for sizing routines: find first available
bus resource of a given type. Note: we intentionally skip the bus resources which have already been assigned (that is, have non-NULL parent resource). */

Fixing the above might be a better solution. But I don't know if there are any users who need it.

I haven't heard of applications which depend on the kernel resource allocation for expansion ROMs.
X is a big user of expansion ROMs but I heard it solves the resource conflict itself.
If there is a counter example, I would like to know it.

Thanks,

Jun'ichi Nomura, NEC Corporation of America

Thats a relief. Jan's report was embarrassing enough. :)

If you are seeing the allocation failures both with and without my original patch I doubt that an improved version would work. In my case, the BIOS had allowed sufficient resource for the expansion ROMs but was expecting the kernel to get it from the non-prefetch instead of prefetch window. In your case, it sounds like there just isn't enough resource available for expansion ROMs from either window.

Like I said above, I doubt that an improved version of my fix would help your situation.

Me too.

Thanks,
Gary

Gary Hade
System x Enablement
IBM Linux Technology Center
503-578-4503  IBM T/L: 775-4503
[email protected]
http://www.ibm.com/linux/ltc

OK. Then it's different from mine.
Thanks for the info.

Regards,

Jun'ichi Nomura, NEC Corporation of America

No problem. Since our goals (eliminate confusing PCI memory allocation failure messages) are the same and since your patch would partially (see below) fix the issue that my patch was attempting to address, I am _very_ interested to see how others who are more expansion ROM knowledgeable than myself react to your proposal. In fact, if what you are saying is correct, I'm wondering why the kernel even needs to attempt (by default) to obtain space for expansion ROMS that on some systems could be better utilized elsewhere. Perhaps the default behavior could be changed to exclude the expansion ROM allocation attempts with a new kernel option added to enable the current behavior for those that might want it. I think this would solve both of our problems.

I will now bore you with my story and how it relates to your change.

The issue that my patch was attempting to address involves
a PCIe adapter that contains a p2p bridge above a SCSI storage
controller:
[root@elm3a9 ~]# lspci -s 0b:00.0
0b:00.0 PCI bridge: PLX Technology, Inc. PEX 8114 PCI Express-to-PCI/PCI-X Bridge (rev bc) [root@elm3a9 ~]# lspci -ts 0b:00.0
-+-[0000:0b]---00.0-[0000:0c]--
\-[0000:00]-
[root@elm3a9 ~]# lspci -s 0c:04.0
0c:04.0 SCSI storage controller: Adaptec ASC-29320ALP U320 (rev 10)

Case 1: (non-hotplug)
Without my patch and in non-hotplug context (adapter
installed at boot time) we see an allocation failure for
the prefetch window that was created due to the BIOS unassigned expansion ROM BAR on the SCSI controller:
PCI: Failed to allocate mem resource #9:100000@f2800000 for 0000:0b:00.0
This is happening because the BIOS allowed space in the non-prefetch window for the expansion ROM and provided
no extra space for the unexpected kernel created prefetch window.

Case 2: (hotplug)
Without my patch and in hotplug context (same adapter hotplugged to the same slot that it was in at boot time)
we see an allocation failure for BAR 0 of the bridge:
PCI: Failed to allocate mem resource #0:2000@f2800000 for 0000:0b:00.0
This is happening because allocations for both the non-prefetch window and expansion ROM motivated prefetch window succeeded consuming all of the memory that the BIOS had provided for the adapter. This left no memory for bridge BAR 0.

Your patch eliminates Case 1 by hiding the prefetch window allocation failure. Case 2 is unfortunately still alive and well because the expansion ROM motivated prefetch window is still consuming the memory that the BIOS provided for bridge BAR 0.

After seeing Jan's report it became obvious that expansion ROMs cannot always be directed to a single type (prefetch or non-prefetch) of window. I know of no direct way of determining where the BIOS expects expansion ROMs to land so some sort of intelligent choice based on other information needs to be made. This is what I have been struggling with. I can handle a simple case where expansion ROM(s) are directed to the non-prefetch window if the calculated non-prefetch window size both with and without expansion ROM(s) is the same. I haven't yet figured out how to handle cases where the with/without expansion ROM non-prefetch window sizes differ. In these cases the BIOS's intention does not appear to be clear. This is why I really like the "skip the default expansion ROM allocation attempts" idea that your proposal has spawned. :)

Thanks for your excellent idea.

Gary

Gary Hade
System x Enablement
IBM Linux Technology Center
503-578-4503  IBM T/L: 775-4503
[email protected]
http://www.ibm.com/linux/ltc