linux-kernel - Re: WARNING at drivers/pci/setup-bus.c:2373, bisected to "PCI: Use pbus_select_window_for

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Message-ID: <af6f0f2e1dec9053c6984139b8582fc6ceab6813.camel@gmail.com>
Date: Sat, 25 Oct 2025 14:44:08 +0200
From: Klaus Kudielka <klaus.kudielka@...il.com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>, 
 Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, regressions@...ts.linux.dev
Subject: Re: WARNING at drivers/pci/setup-bus.c:2373, bisected to "PCI: Use
 pbus_select_window_for_type() during mem window sizing"

On Sat, 2025-10-25 at 12:11 +0200, Klaus Kudielka wrote:
> 
> > [    0.027107] pci 0000:00:03.0: bridge window [mem 0x00200000-0x003fffff] to [bus 02] add_size 200000 add_align 200000
> > [    0.027115] pci 0000:00:03.0: bridge window [mem 0x00200000-0x003fffff] to [bus 02] add_size 200000 add_align 200000
> 
> So, this part of  pbus_size_mem() now seems to be called *TWICE* for the same bridge window:
> 
> 		add_to_list(realloc_head, bus->self, b_res, size1-size0, add_align);
> 		pci_info(bus->self, "bridge window %pR to %pR add_size %llx add_align %llx\n",
> 			   b_res, &bus->busn_res,
> 			   (unsigned long long) (size1 - size0),
> 			   (unsigned long long) add_align);
> 
> 
> 
> WITHOUT the offending commit, I see only one line, and no WARNING.
> > [    0.027405] pci 0000:00:03.0: bridge window [mem 0x00200000-0x003fffff] to [bus 02] add_size 200000 add_align 200000
> 
> 


After some more testing, I think I know what is going on.

- My device seems to have only non-prefetchable IO resources.
- In pci_bus_size_bridges(), pbus_size_mem() is called twice, once with IORESOURCE_PREFETCH, once without.
- This seems to be the intended behaviour (with or without the offending commit).

- What DOES make the difference, is the use of pbus_select_window_for_type() inside pbus_size_mem().
- On my device, that function returns the ***non-prefetchable*** resource, even if being asked for a prefetchable one.
- End result: b_res is valid (and identical) in both calls to pbus_size_mem().
- Honestly, that does not look right to me.


Indeed, my device goes back to the original behaviour (without WARNING), if I go back to the original use of
find_bus_resource_of_type():


--- a/drivers/pci/setup-bus.c
+++ b/drivers/pci/setup-bus.c
@@ -1312,7 +1312,9 @@ static void pbus_size_mem(struct pci_bus *bus, unsigned long type,
        resource_size_t min_align, win_align, align, size, size0, size1 = 0;
        resource_size_t aligns[28]; /* Alignments from 1MB to 128TB */
        int order, max_order;
-       struct resource *b_res = pbus_select_window_for_type(bus, type);
+       struct resource *b_res = find_bus_resource_of_type(bus,
+                       IORESOURCE_MEM | IORESOURCE_PREFETCH | IORESOURCE_MEM_64,
+                       type);
        resource_size_t children_add_size = 0;
        resource_size_t children_add_align = 0;
        resource_size_t add_align = 0;



Comments?