[PATCH 1/2] drivers: pci: do not disregard parent resources starting at 0x0

[Ard Biesheuvel]

On 12 April 2017 at 14:24, Lorenzo Pieralisi <lorenzo.pieralisi at arm.com> wrote:
> [+Yinghai, Bjorn]
>
> On Tue, Apr 11, 2017 at 05:33:12PM +0100, Ard Biesheuvel wrote:
>> Commit f44116ae8818 ("PCI: Remove pci_find_parent_resource() use for
>> allocation") updated the logic that iterates over all bus resources
>> and compares them to a given resource, in order to decide whether one
>> is the parent of the latter.
>>
>> This change inadvertently causes pci_find_parent_resource() to disregard
>> resources starting at address 0x0, resulting in an error such as the one
>> below on ARM systems whose I/O window starts at 0x0.
>>
>> pci_bus 0000:00: root bus resource [mem 0x10000000-0x3efeffff window]
>> pci_bus 0000:00: root bus resource [io  0x0000-0xffff window]
>> pci_bus 0000:00: root bus resource [mem 0x8000000000-0xffffffffff window]
>> pci_bus 0000:00: root bus resource [bus 00-0f]
>> pci 0000:00:01.0: PCI bridge to [bus 01]
>> pci 0000:00:02.0: PCI bridge to [bus 02]
>> pci 0000:00:03.0: PCI bridge to [bus 03]
>> pci 0000:00:03.0: can't claim BAR 13 [io  0x0000-0x0fff]: no compatible bridge window
>> pci 0000:03:01.0: can't claim BAR 0 [io  0x0000-0x001f]: no compatible bridge window
>>
>> While this never happens on x86, it is perfectly legal in general for a
>> PCI MMIO or IO window to start at address 0x0, and it was supported in
>> the code before commit f44116ae8818.
>>
>> So let's drop the test for res->start != 0; resource_contains() already
>> checks whether [start, end) completely covers the resource, and so it
>> should be redundant.
>>
>> Fixes: f44116ae8818 ("PCI: Remove pci_find_parent_resource() use for allocation")
>
> I know this code fixes IO claiming on ARM/ARM64 (well, it fixes nothing
> because we never claim resources on ARM/ARM64 apart from kvmtool and
> generic host bridge), my _big_ worry is that it can cause endless
> regressions on other arches, in any case I would be really really
> careful about adding a Fixes: tag to it.
>

The patch is only 3 years old, and is obviously a regression given
that the change in behavior described here occurs as a side effect.
But given that my involvement with PCI is much more recent than that,
it is very well possible that I am missing something here.

-- 
Ard.